1 00:00:00 --> 00:00:04 This lecture is going to be very dependent upon the PowerPoint slides, 2 00:00:04 --> 00:00:09 because if I take the time to draw the cycles on the board, 3 00:00:09 --> 00:00:14 we'd be here forever. So I would say sit back and relax because you 4 00:00:14 --> 00:00:19 have the slides. They're on the Web. 5 00:00:19 --> 00:00:24 And try to focus on what the take-home message is. 6 00:00:24 --> 00:00:29 OK, but before we go on to biogeochemical cycles, 7 00:00:29 --> 00:00:34 I want to just briefly review some of the things that we learned in the 8 00:00:34 --> 00:00:39 second lecture. I got feedback from many of you, 9 00:00:39 --> 00:00:43 actually, on the things that were difficult to understand from that 10 00:00:43 --> 00:00:48 lecture, and those are important for understanding these cycles. 11 00:00:48 --> 00:00:52 And the one thing that some people were confused by was this anaerobic 12 00:00:52 --> 00:00:57 respiration. Remember, I drew this on the board, 13 00:00:57 --> 00:01:02 and it showed a lot of reduction reactions. 14 00:01:02 --> 00:01:06 And I think that was confusing for some people, so let's just go over 15 00:01:06 --> 00:01:11 that very quickly. You've learned in Graham's lectures, 16 00:01:11 --> 00:01:15 and in my lectures, that anaerobic respiration, 17 00:01:15 --> 00:01:20 respiration of organisms where there is oxygen, that oxygen is the 18 00:01:20 --> 00:01:24 terminal electron receptor here in that electron transport chain and 19 00:01:24 --> 00:01:29 it's reduced to water. So in aerobic environments, 20 00:01:29 --> 00:01:33 when organisms respire oxygen it's reduced to water. 21 00:01:33 --> 00:01:37 If there's no oxygen around, the organisms, and in this case it's 22 00:01:37 --> 00:01:41 always bacteria, look for the next thermodynamically 23 00:01:41 --> 00:01:45 favorable electron acceptor. And so, whatever is dominant in 24 00:01:45 --> 00:01:49 that environment and is most thermodynamically favorable, 25 00:01:49 --> 00:01:53 they'll use. So, the sulfate reducing organisms use sulfate and 26 00:01:53 --> 00:01:57 reduce at [H2S? . Denied trying denitrifying 27 00:01:57 --> 00:02:01 organisms use nitrate and reduce it to these forms of reduced nitrate or 28 00:02:01 --> 00:02:06 reduce nitrogen. And we talked about iron bacteria 29 00:02:06 --> 00:02:10 [epi?] plus three, and reduce it to epi plus two. 30 00:02:10 --> 00:02:14 Some can use manganese, etc. Whatever's there, 31 00:02:14 --> 00:02:18 and it's thermodynamically favorable, they'll use. OK, 32 00:02:18 --> 00:02:22 so that clarifies that. Some of the people said, 33 00:02:22 --> 00:02:26 you kept talking about symmetry. You kept talking about symmetry. 34 00:02:26 --> 00:02:30 I didn't see any symmetry, and in hindsight I can understand why, 35 00:02:30 --> 00:02:35 because it just threw that out and I didn't really point it out. 36 00:02:35 --> 00:02:39 So what I was talking about was as we went through these processes, 37 00:02:39 --> 00:02:44 you see here, these elements, sulfate and nitrogen, 38 00:02:44 --> 00:02:49 nitrate, are being reduced. There are other processes, 39 00:02:49 --> 00:02:53 particularly chemosynthesis in which these reduced compounds, 40 00:02:53 --> 00:02:58 here's H2S and ammonia or be oxidized. So that's the symmetry 41 00:02:58 --> 00:03:03 that I was talking about. And if you didn't have that, 42 00:03:03 --> 00:03:07 if all the organisms were reducing things, the whole system would run 43 00:03:07 --> 00:03:12 down. You have to have organisms that are also oxidizing things. 44 00:03:12 --> 00:03:16 And that's a key component of all of these, or not all of the 45 00:03:16 --> 00:03:21 biogeochemical cycles, but particularly the cycles of 46 00:03:21 --> 00:03:25 nitrogen and sulfur, which have this [redox? 47 00:03:25 --> 00:03:30 chemistry. So that's a symmetry that I was talking about. 48 00:03:30 --> 00:03:36 OK, now so let's talk about how we think about biogeochemical cycles. 49 00:03:36 --> 00:03:42 Did you see the slide in the back? Try to turn the lights off? That 50 00:03:42 --> 00:03:49 was a double question. Can you see the slide in the back? 51 00:03:49 --> 00:03:55 Yes? OK. So this is a generic map of the components of biogeochemical 52 00:03:55 --> 00:04:02 cycles. And we can think of the earth as a giant chemical factory in 53 00:04:02 --> 00:04:08 the sense that has what we call compartments, or reservoirs, 54 00:04:08 --> 00:04:14 or pools of a particular element. Or it might be water that we are 55 00:04:14 --> 00:04:19 analyzing, and then there are fluxes between these pools. 56 00:04:19 --> 00:04:23 So here's a flux, an arrow. So typically these are diagrammed 57 00:04:23 --> 00:04:28 with boxes and arrows connecting them. And you don't have to use all 58 00:04:28 --> 00:04:33 of these boxes. It could be we are just looking at 59 00:04:33 --> 00:04:37 land, atmosphere, and ocean. I mean, 60 00:04:37 --> 00:04:41 you can construct whatever model you want cities. And these are just 61 00:04:41 --> 00:04:46 some useful conversion factors for the amount of things that we're 62 00:04:46 --> 00:04:50 going to have flowing between these compartments. And here, 63 00:04:50 --> 00:04:54 again, this is something we defined earlier when we were talking about 64 00:04:54 --> 00:04:59 productivity. The main residence time of, say, an element, 65 00:04:59 --> 00:05:03 say, carbon in the terrestrial biomass, is the pool size, 66 00:05:03 --> 00:05:07 the amount of carbon that they're divided by the mean flux in or out 67 00:05:07 --> 00:05:12 of the pool, OK? It's exactly the same concept we 68 00:05:12 --> 00:05:16 talked about for carbon. And then the fractional turnover, 69 00:05:16 --> 00:05:20 one over the mean residence time is simply the fraction. 70 00:05:20 --> 00:05:24 If we're talking about carbon again in trees, it's the fraction that's 71 00:05:24 --> 00:05:29 removed per unit time. OK, so you can see that when we talk 72 00:05:29 --> 00:05:34 about mean residence time of an element in one of these reservoirs, 73 00:05:34 --> 00:05:39 if the whole system is in a steady state, in other words, 74 00:05:39 --> 00:05:44 if the amount in a reservoir is changing, the flux in is going to be 75 00:05:44 --> 00:05:50 the same as the flux out, right? You know that. Just like a 76 00:05:50 --> 00:05:55 bathtub, if you have water flowing in, water flowing out, 77 00:05:55 --> 00:06:00 the level will say the same if the flow rate and is the same 78 00:06:00 --> 00:06:05 as the flow rate out. But often in nature, 79 00:06:05 --> 00:06:09 you don't have that exactly. And if you don't, the size of these 80 00:06:09 --> 00:06:13 reservoirs is either increasing or decreasing. So when you're 81 00:06:13 --> 00:06:17 analyzing these systems, your most of the time trying to get 82 00:06:17 --> 00:06:22 the very rough estimate of main residence time. 83 00:06:22 --> 00:06:26 So if the flow in and the flow out isn't the same, 84 00:06:26 --> 00:06:30 you can either average them and use that as your flux, 85 00:06:30 --> 00:06:34 or you could define your residence time with respect to the flow 86 00:06:34 --> 00:06:39 in or the flow out. So, these are just gross 87 00:06:39 --> 00:06:43 approximations. What we want to understand is the 88 00:06:43 --> 00:06:47 residence time thousands of years, millions of years, days, rough 89 00:06:47 --> 00:06:51 approximation. And the other thing I want to say 90 00:06:51 --> 00:06:55 before we go on, is that all of these cycles, 91 00:06:55 --> 00:06:59 we're going to talk about them element by element: phosphorus, 92 00:06:59 --> 00:07:03 carbon, whatever. But they're all tightly coupled in 93 00:07:03 --> 00:07:08 the system. And we'll bring that up again later. Before we go on, 94 00:07:08 --> 00:07:14 let's just look at the solar energy budget, which is driving this whole 95 00:07:14 --> 00:07:19 system, mostly, there is some geothermal energy, 96 00:07:19 --> 00:07:24 energy for the Earth's, the magma that is also driving biogeochemical 97 00:07:24 --> 00:07:30 cycles. But the solar energy is the primary driver. 98 00:07:30 --> 00:07:35 And if you say that the total energy from the sun is 100%, 99 00:07:35 --> 00:07:40 it's that energy that is used in evaporation, and winds, 100 00:07:40 --> 00:07:45 and photosynthesis that is the important component driving the 101 00:07:45 --> 00:07:50 cycles. And you'll see that photosynthesis is a tiny fraction. 102 00:07:50 --> 00:07:56 The energy that plants harvest is a tiny fraction of the total energy 103 00:07:56 --> 00:08:01 that is driving the Earth's system. And yet, this photosynthesis, which 104 00:08:01 --> 00:08:06 is the basis of the biosphere, has an enormous effect on the 105 00:08:06 --> 00:08:11 conditions on Earth. That's an interesting nonlinearity 106 00:08:11 --> 00:08:15 of the system. OK, so let's start with the 107 00:08:15 --> 00:08:19 geological cycle, which is the slowest moving, 108 00:08:19 --> 00:08:24 people don't even think of it as a cycle because while we are on Earth 109 00:08:24 --> 00:08:28 we don't see rocks flying through, well, sometimes you see rocks in a 110 00:08:28 --> 00:08:32 landslide or whatever, but for the most part you don't 111 00:08:32 --> 00:08:37 think of rocks as cycling. But they do. And if they didn't, 112 00:08:37 --> 00:08:41 the system would run down much faster than it is. 113 00:08:41 --> 00:08:45 And we've all heard about plate tectonics, that the surface of the 114 00:08:45 --> 00:08:49 Earth is made up of these plates that are slowly shifting. 115 00:08:49 --> 00:08:53 And when they shift you have earthquakes, like we've had recently 116 00:08:53 --> 00:08:57 a lot of. And also, you have volcanic eruptions that 117 00:08:57 --> 00:09:01 bring material from the inside of the Earth up to the surface 118 00:09:01 --> 00:09:05 and it overflows. And that's part of this geological 119 00:09:05 --> 00:09:09 cycle. So, here's a really oversimplified, 120 00:09:09 --> 00:09:14 when the geology professor in our department so that ever showing this 121 00:09:14 --> 00:09:18 he had a heart attack by how simplified it is. 122 00:09:18 --> 00:09:23 But it's just so you get the idea. When he started editing it, there 123 00:09:23 --> 00:09:27 are so many arrows you can never cope with that. 124 00:09:27 --> 00:09:32 But just get the idea. There's geothermal energy coming in 125 00:09:32 --> 00:09:37 from the inner core of the Earth where you have magma. 126 00:09:37 --> 00:09:41 Think volcanic eruption, lava, which ultimately becomes 127 00:09:41 --> 00:09:45 surface rocks. And they're eroded by weathering by 128 00:09:45 --> 00:09:49 rain, and then elements from that go into the soils. 129 00:09:49 --> 00:09:54 Soils eventually become sedimentary rocks. We're talking over really, 130 00:09:54 --> 00:09:58 really long time periods, which become metamorphic rocks. 131 00:09:58 --> 00:10:02 Some of those are uplifted, and some of them are melted and 132 00:10:02 --> 00:10:07 become magma. But it is a cycle, 133 00:10:07 --> 00:10:12 a very, very slow cycle. In fact, somewhere I read 70%, 134 00:10:12 --> 00:10:18 you do not need to know this. This is not geology class. 135 00:10:18 --> 00:10:23 But just so you have an idea, 75% of the rocks now on the surface 136 00:10:23 --> 00:10:29 of the earth have been uplifted. So it's almost as if the Earth is, 137 00:10:29 --> 00:10:35 on average, maybe halfway through a cycle. So, this erosion here, 138 00:10:35 --> 00:10:42 and weathering, as we talked about last time is critical for making 139 00:10:42 --> 00:10:49 nutrients available to the biosphere. And the force of this weathering is 140 00:10:49 --> 00:10:55 incredibly powerful. One number that I found it one 141 00:10:55 --> 00:11:02 textbook that I never knew before is that Niagara Falls is eroding at 3 142 00:11:02 --> 00:11:09 feet per year. The cusp of the falls from the water 143 00:11:09 --> 00:11:15 is moving back 3 feet per year. That's fast. Another little 144 00:11:15 --> 00:11:22 factoid when I gave this lecture one year, students asked what is going 145 00:11:22 --> 00:11:28 to burn out first on the Earth, the sun or the geothermal energy? 146 00:11:28 --> 00:11:34 And of course I have no idea. We over someday. 147 00:11:34 --> 00:11:38 The Earth is going to be history because without the sun and without 148 00:11:38 --> 00:11:42 the geothermal energy there's no source of energy. 149 00:11:42 --> 00:11:47 So I went to my colleagues in this department, Earth Atmosphere and 150 00:11:47 --> 00:11:51 Planetary Sciences, and said, which is going to burn out 151 00:11:51 --> 00:11:55 first? And they said, roughly the same time. And we have 152 00:11:55 --> 00:12:00 about 2 billion years, so not to worry yet. 153 00:12:00 --> 00:12:04 But it is, we are only here for a period of time. 154 00:12:04 --> 00:12:08 So, that's the geologic cycle. Now, let's move onto the water 155 00:12:08 --> 00:12:12 cycle, and then we're going to go through the element cycles of 156 00:12:12 --> 00:12:17 nitrogen, phosphorus, and carbon. But the water cycle is 157 00:12:17 --> 00:12:21 obviously important in carrying those elements through their cycles. 158 00:12:21 --> 00:12:25 And the cycle is actually fairly well understood. 159 00:12:25 --> 00:12:29 As a fairly because not all of these things, when you're talking 160 00:12:29 --> 00:12:34 about global averages of things it's very difficult. 161 00:12:34 --> 00:12:39 But the weather service is very interested in the global water cycle. 162 00:12:39 --> 00:12:44 So there's been a lot of study done. So in terms of reservoirs, 163 00:12:44 --> 00:12:49 these numbers of black at the amount of water in a reservoir. 164 00:12:49 --> 00:12:54 And the numbers and blue are the number of fluxes annually of the 165 00:12:54 --> 00:13:00 amounts of water moving from one to another. 166 00:13:00 --> 00:13:05 So, there is a lot of water and groundwater. There's a lot of water 167 00:13:05 --> 00:13:10 and ice, and there's a lot of water in the oceans. 168 00:13:10 --> 00:13:15 And there's very little water in the atmosphere. 169 00:13:15 --> 00:13:20 These are the annual fluxes. So you can see, if I animated this 170 00:13:20 --> 00:13:25 right, so 111, these are in terms of square 171 00:13:25 --> 00:13:30 kilometers of water, that's a lot of water. 172 00:13:30 --> 00:13:35 So, 111 minus 71 gives you 40. So that's the rainfall. 173 00:13:35 --> 00:13:39 This is the evapotranspiration, and the net result is 40,000 that is 174 00:13:39 --> 00:13:43 flowing into the oceans. And in the oceans, here's the 175 00:13:43 --> 00:13:48 evaporation and here's the rainfall going in with just a net of 40 176 00:13:48 --> 00:13:52 that's transported from the oceans to land. So you have 40, 177 00:13:52 --> 00:13:56 00 going into the oceans, and 40, 00 coming back, fairly nicely 178 00:13:56 --> 00:14:05 balanced. That's good. And so, let's just use this as an 179 00:14:05 --> 00:14:17 example to say, what's the residence time of water. 180 00:14:17 --> 00:14:30 Let's just calculate this. We can just approximated. 181 00:14:30 --> 00:14:42 So the mean residence time is equal to the pool size divided by the flux, 182 00:14:42 --> 00:14:55 right? So what's the pool size? Well, how much water there in the 183 00:14:55 --> 00:15:06 ocean? Thank you. [35? 184 00:15:06 --> 00:15:15 times 109. And, what's the flux? 185 00:15:15 --> 00:15:31 Well, we have 425, 186 00:15:31 --> 00:15:40 00 evaporating and we have 40 going here. So, I would add this and that 187 00:15:40 --> 00:15:48 so it's balanced. So, I would use 425. 188 00:15:48 --> 00:15:57 4.25 times 105 equals just roughly how many years? 3,000 189 00:15:57 --> 00:16:04 years roughly. So we would say the residence time, 190 00:16:04 --> 00:16:09 the average molecule, the average water molecules floating through 191 00:16:09 --> 00:16:14 this system would spend on average thousands of years in the oceans 192 00:16:14 --> 00:16:18 before it would evaporate and get back into the system. 193 00:16:18 --> 00:16:23 So you should now think about what the average residence time is, 194 00:16:23 --> 00:16:28 for example, in the atmosphere. And you can see when the pool is 195 00:16:28 --> 00:16:33 very small relative to the fluxes, the residence time is going to be 196 00:16:33 --> 00:16:38 very short, right? That's something to remember. 197 00:16:38 --> 00:16:42 When the pool is huge relative to the fluxes, the residence time is 198 00:16:42 --> 00:16:46 going to be very long. So, you should think about that as 199 00:16:46 --> 00:16:50 you go through your notes. But, in oceans, the residence time 200 00:16:50 --> 00:16:55 is thousands of years in groundwater. The residence time, 201 00:16:55 --> 00:16:59 again, can be very long, which is why we don't want to 202 00:16:59 --> 00:17:03 contaminate our groundwater because it's going to take a really long 203 00:17:03 --> 00:17:08 time to flush that through. Lakes: the residence time is on the 204 00:17:08 --> 00:17:13 order of decades, streams on the order of weeks, 205 00:17:13 --> 00:17:19 and atmosphere I'll let you calculate it and figure it out. 206 00:17:19 --> 00:17:24 OK, let's move on now to an element cycle, the global phosphorous 207 00:17:24 --> 00:17:50 cycle. 208 00:17:50 --> 00:17:56 First of all, there's no [redox? chemistry in the cycle. That's 209 00:17:56 --> 00:18:03 important. OK, that's the first thing to remember. 210 00:18:03 --> 00:18:09 And it's called a sedimentary cycle because there is no atmospheric 211 00:18:09 --> 00:18:15 component. There is essentially no phosphorus in the atmosphere. 212 00:18:15 --> 00:18:22 Everything in this field, there's always an exception. 213 00:18:22 --> 00:18:28 There is something called phosphine that comes out of bogs that is 214 00:18:28 --> 00:18:35 really interesting. But it's not a huge amount, 215 00:18:35 --> 00:18:41 so it doesn't really matter in this analysis. And, 216 00:18:41 --> 00:18:47 let's look at it here. We have a fair amount of phosphorus 217 00:18:47 --> 00:18:53 in land plants. And there's internal cycling here. 218 00:18:53 --> 00:19:00 We have the mining of phosphorus from rocks. 219 00:19:00 --> 00:19:04 This is a fertilizer. No, that's not a mine; that's a 220 00:19:04 --> 00:19:08 house. Sorry. The mine is invisible. 221 00:19:08 --> 00:19:12 Here's the mine. So, the phosphorus is being mined. 222 00:19:12 --> 00:19:17 It's put on crops as fertilizer. The crops are eaten by the people 223 00:19:17 --> 00:19:21 in the house, and the phosphorus ends up in sewage. 224 00:19:21 --> 00:19:25 Even if it's treated, it ends up in the rivers, 225 00:19:25 --> 00:19:30 and it ends up flowing into the oceans. 226 00:19:30 --> 00:19:35 And there's a little bit in dust transport here, 227 00:19:35 --> 00:19:40 but if you look at this whole system, it's basically the phosphorous cycle 228 00:19:40 --> 00:19:46 is a one-way flow to the oceans. The only return of the cycle is via 229 00:19:46 --> 00:19:51 the sedimentary cycle where you go from sediments. 230 00:19:51 --> 00:19:57 Those are sedimentary rocks until you go to mineable rock 231 00:19:57 --> 00:20:02 and through uplifting. And this is on geological timescales. 232 00:20:02 --> 00:20:08 So, on the earth today, the global phosphorous cycle is 233 00:20:08 --> 00:20:13 really not a cycle. It's a one-way flow to the oceans. 234 00:20:13 --> 00:20:18 Well, it's a cycle, but it's an extremely unbalanced cycle because 235 00:20:18 --> 00:20:24 eventually this one will come back. It cycles very rapidly in the biota, 236 00:20:24 --> 00:20:29 internal cycling in the ocean. So, it comes of the river, it's 237 00:20:29 --> 00:20:34 taken up by phytoplankton, there you can buy zooplankton, 238 00:20:34 --> 00:20:40 and then the phosphorus is excreted or bacteria [UNINTELLIGIBLE] on dead 239 00:20:40 --> 00:20:49 organisms take up the phosphorous. It's excreted as organic phosphorous, 240 00:20:49 --> 00:21:02 and it cycles rapidly through this system. OK. 241 00:21:02 --> 00:21:15 So the other important feature of this one way flow, and also 242 00:21:15 --> 00:21:33 humans have altered. 243 00:21:33 --> 00:21:40 In other words, humans are responsible for this, 244 00:21:40 --> 00:21:47 basically, one-way flow by mining the phosphorus and putting it into 245 00:21:47 --> 00:21:54 the agricultural system. OK, yeah, and there's the return 246 00:21:54 --> 00:22:01 flux. OK, moving on to the nitrogen cycle, 247 00:22:01 --> 00:22:07 which is much more complicated because it is [redox? 248 00:22:07 --> 00:22:14 chemistry, OK? And, humans have also had a major, 249 00:22:14 --> 00:22:20 major, major effect on the global nitrogen cycle. 250 00:22:20 --> 00:22:27 So, let's first look at the global estrogen transformation. 251 00:22:27 --> 00:22:31 So this isn't a pools and fluxes diagram. This is a summary for you 252 00:22:31 --> 00:22:36 of things you already know. You already know this. It just 253 00:22:36 --> 00:22:40 looks different than what you learned in the second lecture. 254 00:22:40 --> 00:22:45 So let's just go through it very quickly. If we think of the 255 00:22:45 --> 00:22:50 compounds of nitrogen as being either reduced or oxidized and we 256 00:22:50 --> 00:22:54 think of the environment where they might be found as either being 257 00:22:54 --> 00:22:59 aerobic or oxic, having oxygen, or anaerobic and oxic, 258 00:22:59 --> 00:23:05 not having oxygen. We can draw a schematic of these 259 00:23:05 --> 00:23:11 processes that hopefully makes good sense. If we start with organic 260 00:23:11 --> 00:23:17 nitrogen, but say it's a dead whale that you saw is organic nitrogen, 261 00:23:17 --> 00:23:23 bacteria work on it, and through this process which you haven't 262 00:23:23 --> 00:23:29 really learned about explicitly at, can convert that to free ammonia. 263 00:23:29 --> 00:23:34 That ammonia can be used in chemosynthesis, 264 00:23:34 --> 00:23:40 which you learned about. OK, the specific type of 265 00:23:40 --> 00:23:45 chemosynthesis is called nitrification, 266 00:23:45 --> 00:23:51 where this ammonia is converted to nitrite. Is that an oxidation or a 267 00:23:51 --> 00:23:56 reduction? Shout it out. Yes, yes. It's an oxidation. 268 00:23:56 --> 00:24:02 This was obvious because you can actually see the oxygen. 269 00:24:02 --> 00:24:06 So, in that, nitrite also in chemosynthesis can be further 270 00:24:06 --> 00:24:11 oxidized to nitrate. And chemosynthesis, so this is an 271 00:24:11 --> 00:24:16 energy releasing process for these bacteria. Now, 272 00:24:16 --> 00:24:20 so here we now have nitrogen in an oxidized form, 273 00:24:20 --> 00:24:25 and we are in an anoxic environment, and that should immediately tell you, 274 00:24:25 --> 00:24:30 oh, that's an electron acceptor for the anaerobic bacteria which are 275 00:24:30 --> 00:24:35 going to dump their electrons on this and convert it to NO or N2O. 276 00:24:35 --> 00:24:40 These are gases, and nitrogen gas. 277 00:24:40 --> 00:24:45 This is denitrification or anaerobic respiration, 278 00:24:45 --> 00:24:50 which we already talked about. And it also can be converted 279 00:24:50 --> 00:24:55 through nitrogen fixation, N2 gas can be converted to ammonia. 280 00:24:55 --> 00:25:00 We already talked about this, too. 281 00:25:00 --> 00:25:04 Remember, bacteria and cyanobacteria are the only organisms that can take 282 00:25:04 --> 00:25:09 nitrogen gas from the atmosphere and converted to ammonia for the use of 283 00:25:09 --> 00:25:13 other organisms. OK, and then there's one other 284 00:25:13 --> 00:25:18 thing here which is called assimilatory nitrate reduction. 285 00:25:18 --> 00:25:22 And that is when organisms just take up nitrate, 286 00:25:22 --> 00:25:27 and inside them, and they reduce it so that they can, 287 00:25:27 --> 00:25:31 they have to reduce it to ammonia in order to reduce it for 288 00:25:31 --> 00:25:38 protein synthesis. So that's another route for nitrate 289 00:25:38 --> 00:25:46 to become organic nitrogen in an oxidized environment. 290 00:25:46 --> 00:25:54 So these are the important biological transformations in the 291 00:25:54 --> 00:26:03 cycle. So, here's the cycle in all of its complexity. 292 00:26:03 --> 00:26:10 And redox is important. That's a feature. I'm going to 293 00:26:10 --> 00:26:18 list these things, and then we'll look at them on the 294 00:26:18 --> 00:26:25 diagram, has a gaseous phase, in other words, is an important 295 00:26:25 --> 00:26:33 atmosphere component, N2, NO, N2O, and by the way, 296 00:26:33 --> 00:26:41 this is a very powerful greenhouse gas. 297 00:26:41 --> 00:26:52 So, the balance or imbalance in the nitrogen cycle that results in more 298 00:26:52 --> 00:27:03 or less N2O is very important for global climate regulation. 299 00:27:03 --> 00:27:29 Nitrogen fixation by microbes and humans: very important. 300 00:27:29 --> 00:27:37 And denitrification by microbes is the only way to return nitrogen to 301 00:27:37 --> 00:27:45 the atmosphere. If you didn't have denitrification, 302 00:27:45 --> 00:27:54 this process that you learned in my second lecture that you thought was 303 00:27:54 --> 00:28:02 just some weird way things get through life, is incredibly 304 00:28:02 --> 00:28:11 important in maintaining the global nitrogen cycle. 305 00:28:11 --> 00:28:18 So, let's look at this, the details here. So, in terms of 306 00:28:18 --> 00:28:26 nitrogen fixation, that's taking N2 gas and converting 307 00:28:26 --> 00:28:33 it to ammonia. Biological fixation by plants, 308 00:28:33 --> 00:28:39 or it's really not by plants. It's by the symbiotic microbes in their 309 00:28:39 --> 00:28:46 roots is 140 times 1012 of grams per year. The industrial electrician 310 00:28:46 --> 00:28:52 fixation, that is, what's done by humans, 311 00:28:52 --> 00:28:59 there's a process called the Haber process that's incredibly energy 312 00:28:59 --> 00:29:05 intensive. It takes a lot of fossil fuel to break that nitrogen 313 00:29:05 --> 00:29:11 triple bond. In other words, 314 00:29:11 --> 00:29:15 to take nitrogen gas and convert it to ammonia, you have to break this 315 00:29:15 --> 00:29:19 triple bond which is very energy intensive. But they figured that 316 00:29:19 --> 00:29:23 out during World War II basically, or was it World War I? Anyway, one 317 00:29:23 --> 00:29:27 of the wars, how to break that bond, and that was the beginning of the 318 00:29:27 --> 00:29:32 nitrogen fertilizer industry. So, this is human nitrogen fixation 319 00:29:32 --> 00:29:38 that is used to fertilize crops. So this is a huge fraction of the 320 00:29:38 --> 00:29:43 natural fixation. I mean, this adds a huge amount of 321 00:29:43 --> 00:29:49 nitrogen flux to the system. OK, in this flux here, this is 322 00:29:49 --> 00:29:54 cultivated legume. So, this would be agricultural bean 323 00:29:54 --> 00:30:00 plants that naturally have nitrogen fixers in them, 324 00:30:00 --> 00:30:06 and that also import nitrogen into the system. 325 00:30:06 --> 00:30:12 So, we consider that part of the human flux. OK, 326 00:30:12 --> 00:30:18 to balance this, we have denitrification, 327 00:30:18 --> 00:30:25 which as I said is done by microbes on land and in the ocean. 328 00:30:25 --> 00:30:31 So, looking at this, is it balanced? Is nitrogen fixation on a global 329 00:30:31 --> 00:30:37 scale and denitrification balanced? Did I hear a no? 330 00:30:37 --> 00:30:43 Which is greater? Denitrification, yeah. 331 00:30:43 --> 00:30:48 If you add this, this, and this, you get 260. Is that right? Yeah, 332 00:30:48 --> 00:30:54 and then you add this, this, and this you get 310. 333 00:30:54 --> 00:31:00 So, there's more nitrogen going into the atmosphere than 334 00:31:00 --> 00:31:06 we're taking out. And people don't understand this. 335 00:31:06 --> 00:31:13 They think the denitrification has been disproportionately stimulated 336 00:31:13 --> 00:31:19 by this huge flux of nitrogen into the system. But this is an 337 00:31:19 --> 00:31:26 important imbalance that a lot of people are studying very 338 00:31:26 --> 00:31:32 hard. OK, yeah. That's the major feature that you 339 00:31:32 --> 00:31:36 want to look at in the system. And then, if we compare, this 340 00:31:36 --> 00:31:41 figure is from your textbook comparing the biological nitrogen 341 00:31:41 --> 00:31:45 fixation. Plus, lightning fixes it a little bit. 342 00:31:45 --> 00:31:50 Compared to the human, you can see that humans are now responsible for 343 00:31:50 --> 00:31:54 an equal amount of nitrogen flux on a global scale as the natural system. 344 00:31:54 --> 00:31:59 This is a dramatic perturbation, and that's only in the last 50 years 345 00:31:59 --> 00:32:04 or so, dramatic perturbation to the system. 346 00:32:04 --> 00:32:09 This amount that we are doing is, 140 gigatons is equivalent to 10 347 00:32:09 --> 00:32:15 million trucks of dry nitrogen fertilizer that we are putting into 348 00:32:15 --> 00:32:20 the system with completely unknown effects. OK, the next series of 349 00:32:20 --> 00:32:26 slides are just to illustrate in one ecosystem the importance of the 350 00:32:26 --> 00:32:32 biota and maintaining nitrogen in the ecosystem. 351 00:32:32 --> 00:32:35 And I'll also just show you the importance of experiments in ecology. 352 00:32:35 --> 00:32:39 And this is the Hubbard Brook Experimental Forest, 353 00:32:39 --> 00:32:43 which is up in New Hampshire. Some of you might have even visited 354 00:32:43 --> 00:32:47 there. This was my first job as a graduate student was actually 355 00:32:47 --> 00:32:50 working in the forest. I was measuring phosphorous 356 00:32:50 --> 00:32:54 concentrations in the streams. And what they do there, just like 357 00:32:54 --> 00:32:58 that experimental lake study I showed you, here, 358 00:32:58 --> 00:33:02 they have permits from the forest service to clear cut 359 00:33:02 --> 00:33:06 entire watersheds. A watershed is just an area that 360 00:33:06 --> 00:33:10 collects the rainfall and directs it into a single stream. 361 00:33:10 --> 00:33:14 You can collect the rain and measure what's in it, 362 00:33:14 --> 00:33:18 and you can collect the water coming out and measure what's in it. 363 00:33:18 --> 00:33:22 And the difference is what the ecosystem is actually doing. 364 00:33:22 --> 00:33:26 So, what they did was they had these two watersheds that were the 365 00:33:26 --> 00:33:30 same, and they clear cut one of them. And they asked with the influence 366 00:33:30 --> 00:33:34 of this clear cutting was on the quality of the water coming 367 00:33:34 --> 00:33:38 out of the system. And to make a long story very short, 368 00:33:38 --> 00:33:42 it's a really fascinating study that's been going on for four years 369 00:33:42 --> 00:33:47 that I want to tell you because then you'll know how old I am. 370 00:33:47 --> 00:33:51 But, here's the control watershed, and here's the water coming out of 371 00:33:51 --> 00:33:56 the devegetated one, showing massive [reflux? 372 00:33:56 --> 00:34:01 of nitrate from the system as well as other cations. 373 00:34:01 --> 00:34:04 And your textbook does a terrible job of not explaining this. 374 00:34:04 --> 00:34:08 And I don't have time to go into the details. But, 375 00:34:08 --> 00:34:12 the major reason this is lost, the vegetation is really important 376 00:34:12 --> 00:34:16 in that, but it's important in also maintaining the microbial community 377 00:34:16 --> 00:34:20 in the soil. And when it's cut down, the microbial community changes. 378 00:34:20 --> 00:34:24 And that is very important and resulting in the loss. 379 00:34:24 --> 00:34:28 It's a beautiful study, which unfortunately we don't have 380 00:34:28 --> 00:34:32 time to go into. But if you're interested, 381 00:34:32 --> 00:34:36 I can point you in the right direction. OK, 382 00:34:36 --> 00:34:40 now, let's go into the really important, well, 383 00:34:40 --> 00:34:44 they're all important because they're all coupled. 384 00:34:44 --> 00:34:48 But this is the one that's getting a lot of attention, 385 00:34:48 --> 00:34:52 the global carbon cycle. And it's getting a lot of attention 386 00:34:52 --> 00:34:56 because we have had an incredibly significant impact on it, 387 00:34:56 --> 00:35:01 and we are worried about that causing major global warming. 388 00:35:01 --> 00:35:05 And as an aside, I'll just tell you that I actually 389 00:35:05 --> 00:35:10 think the global nitrogen cycle is a sleeping giant, 390 00:35:10 --> 00:35:14 and that the public doesn't know much about that right now. 391 00:35:14 --> 00:35:19 But in the scientific community, we know the perturbation we've had 392 00:35:19 --> 00:35:23 on that cycle could end up being equally, if not more, 393 00:35:23 --> 00:35:28 traumatic for the Earth's climate as this. But that's an aside. 394 00:35:28 --> 00:35:33 So let's focus on this now. So, here's the global not carbon 395 00:35:33 --> 00:35:38 cycle, which you've seen now several times in my lectures. 396 00:35:38 --> 00:35:43 So here's gross primary productivity, and respiration by 397 00:35:43 --> 00:35:48 land plants, respiration by the soils. These are RA and RH that we 398 00:35:48 --> 00:35:53 talked about before. And in this, we have their balance, 399 00:35:53 --> 00:35:58 roughly balanced, and then you have uptake by the oceans, 400 00:35:58 --> 00:36:03 and loss of CO2 by the oceans. Your textbook says this is all a 401 00:36:03 --> 00:36:07 physical and chemical process that's absolutely wrong. 402 00:36:07 --> 00:36:11 The biota are central to that, and that's another lecture. But you 403 00:36:11 --> 00:36:15 already know that, that the phytoplankton are sucking a 404 00:36:15 --> 00:36:20 lot of CO2 in through photosynthesis. So let's look at the budget here. 405 00:36:20 --> 00:36:24 And, this is the introduction of CO2 into the atmosphere by burning 406 00:36:24 --> 00:36:28 fossil fuel, and the introduction of CO2 into the atmosphere by 407 00:36:28 --> 00:36:34 destruction of vegetation. So, we have 7. 408 00:36:34 --> 00:36:42 gigatons going into the atmosphere due to human perturbation. 409 00:36:42 --> 00:36:49 The annual increase of CO2 in the atmosphere is 3. 410 00:36:49 --> 00:36:57 gigatons. So, 3. gigatons annual increase, 411 00:36:57 --> 00:37:03 and, let's see. If we look at the difference here 412 00:37:03 --> 00:37:07 between respiration and photosynthesis we see that there's 2 413 00:37:07 --> 00:37:11 gigatons going into the vegetation, actually net into the vegetation. 414 00:37:11 --> 00:37:15 And if we look at this, we see that there's two going into 415 00:37:15 --> 00:37:19 the ocean. So, if we ask, of all of this 416 00:37:19 --> 00:37:23 anthropogenic CO2 where's it going? 3.5 is going to increase in the 417 00:37:23 --> 00:37:27 atmosphere. Two is going to vegetation, and two is 418 00:37:27 --> 00:37:33 going to the ocean. And it's this that we are very 419 00:37:33 --> 00:37:39 concerned about because it's causing a dramatic increase in the CO2 in 420 00:37:39 --> 00:37:45 the atmosphere. Even though these are tiny fluxes 421 00:37:45 --> 00:37:51 relative to the global biological fluxes, these tiny fluxes lead to a 422 00:37:51 --> 00:37:57 significant increase because the pool is so small of CO2 423 00:37:57 --> 00:38:02 in the atmosphere. So, this is a trace of CO2 since 424 00:38:02 --> 00:38:06 1960. Here's a question for you to think about. I'm not going to 425 00:38:06 --> 00:38:11 answer it. Its summer and winter are out of phase in the Northern and 426 00:38:11 --> 00:38:15 Southern Hemisphere, why isn't this just smooth? 427 00:38:15 --> 00:38:19 This cycle that we see here is an annual cycle of the Earth breathing. 428 00:38:19 --> 00:38:24 Remember I showed you that the first lecture showing photosynthesis 429 00:38:24 --> 00:38:28 greater than respiration during the summer, and the reverse 430 00:38:28 --> 00:38:33 during the winter. Think about why it isn't just smooth 431 00:38:33 --> 00:38:38 and canceled out by the two hemispheres. OK, 432 00:38:38 --> 00:38:42 and if we look at that same graph, this is atmospheric CO2 from ice 433 00:38:42 --> 00:38:47 core data as a function of time. This is today, and this is time 434 00:38:47 --> 00:38:52 before present going backwards. This is 450,000 years ago. We can 435 00:38:52 --> 00:38:56 see that CO2 in the atmosphere, and this is measured in, you take a 436 00:38:56 --> 00:39:01 deep ice core in Greenland, or something, and you measure the 437 00:39:01 --> 00:39:06 CO2 concentration at different slices of the core. 438 00:39:06 --> 00:39:11 And it tells you what the Earth was like back then. 439 00:39:11 --> 00:39:16 And, this just dramatically shows you what we are doing just that the 440 00:39:16 --> 00:39:21 last hundred years. We have increased CO2 in the 441 00:39:21 --> 00:39:26 atmosphere dramatically by burning fossil fuels. And CO2 is a 442 00:39:26 --> 00:39:31 greenhouse gas, and so we are very concerned about 443 00:39:31 --> 00:39:36 that. OK, this is just read showing that 444 00:39:36 --> 00:39:40 slide from last time of upwelling to remind you that the biogeochemical 445 00:39:40 --> 00:39:45 cycles of these elements are tightly coupled. Remember, 446 00:39:45 --> 00:39:49 we talked about nutrients, nitrogen, phosphorus, being upwelled 447 00:39:49 --> 00:39:54 from the deep water, phytoplankton taking them up, 448 00:39:54 --> 00:39:58 drawing down CO2 and then we had oxygen and CO2 going back 449 00:39:58 --> 00:40:03 and forth in the water. So, the oxygen cycle, 450 00:40:03 --> 00:40:09 which we haven't even talked about is tightly coupled also to the CO2 451 00:40:09 --> 00:40:14 cycle. I'm not going to show this there. OK, moving on, 452 00:40:14 --> 00:40:19 and I know this is quick, but this is in your readings. 453 00:40:19 --> 00:40:25 There is a newspaper article about the Biosphere 2 experiment which now 454 00:40:25 --> 00:40:30 is pretty dated. To make a long story short, 455 00:40:30 --> 00:40:36 many years ago a very rich person built the system out in the middle 456 00:40:36 --> 00:40:41 of the Arizona desert. And it had seven ecosystems in it. 457 00:40:41 --> 00:40:45 It was sealed. It was closed. And, he put people at, 458 00:40:45 --> 00:40:49 which were called biospherians, and the idea was to see whether 459 00:40:49 --> 00:40:54 humans could create a closed biosphere that would sustain human 460 00:40:54 --> 00:40:58 life. And, it was a miserable failure, which is sad because it 461 00:40:58 --> 00:41:02 costs a lot of money, and has since been taken over by 462 00:41:02 --> 00:41:07 Columbia University to use it as an experimental facility. 463 00:41:07 --> 00:41:11 But the one thing that they learned, here's what happened. They put the 464 00:41:11 --> 00:41:16 people in. And it turned out that there was not enough photosynthesis 465 00:41:16 --> 00:41:21 to supply enough oxygen for the people to breathe. 466 00:41:21 --> 00:41:25 Oxygen levels steadily went down. And the reason for that, they 467 00:41:25 --> 00:41:30 learned later, was that they had put way too much 468 00:41:30 --> 00:41:35 rich soil in the system. So, the bacteria in the soil were 469 00:41:35 --> 00:41:39 sucking the oxygen out of the atmosphere. And there were 470 00:41:39 --> 00:41:44 subsidizing the system with rich soil so that people would have 471 00:41:44 --> 00:41:49 enough food. But there was a puzzle, because if this was the case, 472 00:41:49 --> 00:41:53 because the cycles are coupled, you should expect to see the same 473 00:41:53 --> 00:41:58 amount of, if this oxygen is coming from photosynthesis, 474 00:41:58 --> 00:42:03 you should see the same amount of CO2 coming into the system. 475 00:42:03 --> 00:42:07 And you should see an increase in CO2 in the atmosphere. 476 00:42:07 --> 00:42:11 And they didn't. In other words, they saw oxygen going down, but they 477 00:42:11 --> 00:42:16 didn't see surplus CO2 in the atmosphere. And it took a bright 478 00:42:16 --> 00:42:20 graduate student from Columbia University to go in there and figure 479 00:42:20 --> 00:42:25 out what was going on. And it turned out that, 480 00:42:25 --> 00:42:29 so why didn't CO2 increase? It turned out that this CO2, 481 00:42:29 --> 00:42:34 which was coming out of the system from respiration in the soil was 482 00:42:34 --> 00:42:38 actually binding to the calcium hydroxide in the cement and making 483 00:42:38 --> 00:42:43 calcium carbonate. So, the cement, 484 00:42:43 --> 00:42:47 another human invention, was playing an important role here. 485 00:42:47 --> 00:42:51 The point is that none of this, this is only understandable in 486 00:42:51 --> 00:42:55 hindsight, because it didn't work. You can go in and figure out, what 487 00:42:55 --> 00:42:59 the heck, where did these imbalances come from? So, 488 00:42:59 --> 00:43:03 it was a very interesting study, and we learned that it's not easy to 489 00:43:03 --> 00:43:08 mimic natural biosphere on a very small scale. 490 00:43:08 --> 00:43:12 OK, I'm going to skip that one, and come to this real quickly, 491 00:43:12 --> 00:43:17 because this was just on the news this morning as I was driving into 492 00:43:17 --> 00:43:22 work. I thought, perfect for this lecture. 493 00:43:22 --> 00:43:27 The UN just announced this millennium ecosystem assessment. 494 00:43:27 --> 00:43:32 It's on the web. And 2,000 scientists have been 495 00:43:32 --> 00:43:36 working on this for over ten years trying to assess the state of the 496 00:43:36 --> 00:43:40 global ecosystems and their capability to support future 497 00:43:40 --> 00:43:44 generations, i. . you guys. And they say the next 498 00:43:44 --> 00:43:49 50 years, and those are the 50 years that you guys are in charge, 499 00:43:49 --> 00:43:53 are absolutely critical for whether or not these systems will sustain, 500 00:43:53 --> 00:43:57 be able to sustain human populations. So you can go to the web if you're 501 00:43:57 --> 00:44:02 interested in that. OK, quickly to our civil and 502 00:44:02 --> 00:44:06 environmental engineering major, I'm just going to say that our new 503 00:44:06 --> 00:44:11 motto is nature, tools, and toys, that nature is 504 00:44:11 --> 00:44:16 ecology. There is a two series ecology course. 505 00:44:16 --> 00:44:20 Tools are mechanics: basics, fundamentals for analyzing systems. 506 00:44:20 --> 00:44:25 And toys is design. The part of the curriculum is going to be 507 00:44:25 --> 00:44:30 designing instrumentation for studying environmental systems. 508 00:44:30 --> 00:44:35 And there are these brochures here and in the back. 509 00:44:35 --> 00:44:40 So I encourage you to pick those up if you're at all interested in that 510 00:44:40 --> 00:44:45 major. Now, let me show you this cool clip. Don't leave yet. 511 00:44:45 --> 00:44:51 This is worth it. It's only two minutes, and its nature at its best. 512 00:44:51 --> 00:44:56 So, all I need to do here is hit play. And this is the soccer 513 00:44:56 --> 00:45:02 player's look like this. Oh, why didn't that work. 514 00:45:02 --> 00:45:22 [Miophon?] our little 515 00:45:22 --> 00:45:51 bugs in the sand. 516 00:45:51 --> 00:46:08 That's my favorite part. 517 00:46:08 --> 00:46:21 Life is a geological agent. 518 00:46:21 --> 00:46:25 See, that would have been a great kick off for spring break, 519 00:46:25 --> 00:46:30 but welcome back from spring break. All right, I'll see you in a few 520 00:46:30 --> 00:46:33 weeks.