Full net accounting for C and N uptake (input) and fate in grazed pastures
Carbon balance (dairy system)
The diagrams (stacked bars) depict the gross photosynthetic input of C (in units kg C), and the breakdown of the fate of that carbon, in grazed pastures, for the combinations of circumstances described on the bottom axes. These correspond exactly to where those circumstances are described on other pages (eg see graphs ‘a’ to ‘g’ under ‘irrigation’) and so the graphs below expand on the points made there (and vice versa).
Note, we are describing then the C (and later N) balances of systems in their sustainable ‘steady state’. All ‘states’ fluctuate over the seasons, and those seasonal variations could be drawn, but what we describe, here, is the annual C balance, sustained and sustainable, year on year. The top of each bar is the rate of gross photosynthesis* (C input to the system), and the subdivisions within each bar account completely for its fate. Note there are no ‘states’ listed, eg the amount of C ‘sequestered’ in soil organic matter, or ‘root mass’ etc, nor should there be. The accumulation of C into such ‘states’ takes place during transitions (and so is ‘transient’ ). For a full description of that see ‘clarifications’. And for an analysis for just how much C was sequestered in the system, see graph ‘b’ under ‘irrigation’. We are describing, here, a sustainable ‘steady state’, at which C is certainly being input to eg ‘soil organic matter’ and ‘root mass’, and these are ‘turning over’ (some components on extremely long time scales). The net change in the ‘states’ per se is therefore zero. The C accounting includes all rates of input (to the system as a whole) and all rates of output from the system as a whole.
Full descriptions of all components of the internal recycling of C and N will be attempted elsewhere.. the challenge is more one of how to encompass so much in any one picture!
The first graph below accounts for all the C uptake and fate for a ‘dairy’ system (hence ‘L’ for ‘lactation’), receiving fertiliser N inputs of 15, 30, 60, 150, 300, 600 kg N/ha/year, and in this example for an un-irrigated pasture (the ‘dry’ here is the same data set as for ‘dry’ on the graphs under the section ‘irrigation’). The predictions are based on Winchmore soil type and met data, which is typified by there being at least 3 months in summer during which water deficit depresses photosynthesis (the model considers this in full physiological detail). Hence ‘dry’ is used for ‘un-irrigated’ and for brevity. In winter the system may, yet, have excess water.
This second graph is for the same system, but receiving extra water, ‘irrigation’, during the critical summer months. Water is added to the minimum extent required to ‘just’ relieve the physiological constraints (water deficit) to photosynthesis. In practice this was close to twice the natural rainfall amount during those critical periods. ‘Wet’ here is again the same case precisely as elsewhere, eg under ‘irrigation’.
The addition of fertiliser N, and of water, had the greatest effects on the amounts (rate per ha per year) of photosynthesis and so C inputs and releases. It had relatively little effect on the proportional breakdown of the fate of carbon. Note this is in distinct contrast to the fate of N, see below.
N balance (dairy system)
The input and fate of N in the precise same systems as used in the example above, are shown here…
Again, the top of the bar in each case is the total gross input of N to the system which here represents N fertiliser inputs, (of 15, 30, 60, 150, 300, 600 kg N/ha/year) and a small ‘wet and dry atmospheric deposition’ of N (just 2 kgN/ha/year). The model can consider legume N inputs and of course inputs of C and N from supplement, but for clarity the example used here is where N inputs are predominantly fertiliser. Note again, as for the C cycle, some major fluxes are not shown, eg urine N flows to the soil, because these are NOT inputs to the system (see ‘clarifications’), but components of internal recycling within the system. Rest assured all graphs drawn in this web-site include all such components of internal recycling, and all the phenomena that result from those fluxes are described.
The inputs of N to the system, and the nature of the outputs of N from the system are described first, below, for an unirrigated dairy system..
and then for the otherwise identical but now ‘irrigated’ system (water added in summer at critical times to ‘just’ relieve physiological constraints to photosynthesis)…
Explanation… walk through….. (couple of paragraphs)
Proportional fate of C and N
The input of fertiliser (looking left to right across the bars) increased the proportion of C inputs that was harvested as milk, as would be completely anticipated, albeit note this applied only up to c. 150 kg N/ha/year inputs. To see this more clearly, see Figure (a) ‘food production (kg protein_N/ha/year) under ‘irrigation’. The value of seeing this in the context of all other flows of C is to re-inforce how small a proportion of the total flow of carbon, the yield of C in products represents. Even smaller (in this context) is the amount and proportion of C emitted as methane. The proportion is so small it is almost not visible on the graphs below. This does NOT deny its significance as a potent GHG emission. To see the absolute emissions or methane per ha, plotted alone on an axis, see graph ‘d’ under ‘irrigation’.
These ‘proportionality’ graphs (below) strongly re-inforce how nearly all of the carbon taken up in photosynthesis is, of course, released in respiration. As above, the largest component of the return of C to the atmosphere, is in soil respiration (alone some 40%), which is greater when one adds in ‘root respiration’ (summing to some 55-60% of total C releases. Plant respiration (see root respiration and add in shoot respiration) is some 45-50% of total releases to the atmosphere. The animal (above ground) respiration is some 5-10% to that total.
These proportionalities (in the fate of Carbon) are little affected by the addition of water (irrigation).
The ‘story’ is very different for the fate of N (below)
Comparison with meat system?