Irrigation and how it interacts with fertiliser N input and animal class


















The response to nitrogen fertiliser N input (x axis) of food production (a); the total amount of C sequestered (b); the total rate of release of N to the environment (c) and the rate of emission of methane (d), all expressed per hectare. The solid line depicts ‘meat’ systems (cattle or sheep in general), and the dashed lines depict ‘dairy’ systems (which could equally be either cattle or sheep lactation based). The points (symbols) along each line are the solutions for each of a range of fertiliser N input rates (of 15, 30, 60, 150, 300, and 600 kgN/ha/year).

All outputs here are based on the met data and soil type for Winchmore (S Island, NZ), where despite sustained water inputs from rain, there is insufficient water (a water deficit develops that limits photosynthesis) during three months each summer. Hence the unirrigated treatment is labelled ‘dry’; ‘wetter’ is where some 50% more water has been added during each of the three critical summer months; and ‘wet’ is where sufficient water has been added in those months, to just relieve water deficit depressing photosynthesis (in practice some 100% more.. hence a doubling.. of rain water inputs).

All are the long-term sustainable rates and states predicted by the model, noting that outcomes in the shorter term, including then those even from direct experiments/empirical measurements, can reflect temporary transients, some which run counter to what the final outcome will be after the whole system has re-adjusted (see publications). All are based on stocking rate being optimally matched to the rate of supply of fresh forage (see ‘clarifications’). This overcomes the scatter and distraction of simple ‘mis-management’ of pasture.


Interaction between water input and nitrogen input

As spelt out on the page ‘C sequestration’ , water and nitrogen can each act to remove major constraints to plant growth, physiologically, notably by substantially increasing carbon capture by photosynthesis. However, the two limiting factors will interact, depending whether the plant system is (at the time) C limited or N limited. Hence, responses to N will be limited if there is insufficient water to prevent water vapour deficit restricting photosynthesis, and likewise there will be limited response to water if the plant system lacks available nitrogen. So much is simple.

Hence, as in Figure (a) above, a very substantial increase in C capture, and so plant growth, intake and yield of products per ha, arises when water is added during a period of otherwise restricted photosynthesis. The increase in C capture, and yield of products, is so substantial in this example, following irrigation, because the addition of water basically adds an extra three-four months of C capture over the season at this site (Winchmore, NZ).  Photosynthesis and plant growth is restricted during 3-4 months over summer because the high light energy receipt, during those months, leads to a major water vapour deficit, and restriction of photosynthesis, despite rainfall in those months being similar to other times in the year.

The interaction with nitrogen input is evident in that the relative increase in the yield of products is greater at high nitrogen input rates (where the system is less N-limited) than at low N input rates.

The major effect of irrigation in increasing C capture is evident further in the similarly major increases in methane release per ha (as this reflects animal intake per ha, which in turn, working backwards, reflects  greater plant growth per ha) see (d).

Note, that despite the greater C capture sustained under irrigation, this does not lead to a major increase (nor note a major loss) in C sequestration, when compared at the same  N input rates, see (b).  The reason why an increased C capture does not (in this model) lead to increased C sequestration, is as follows.

The increased C capture, led, recall, to a sustainably far greater removal of both C and N in animal products, as seen in (a). Hence at any given fixed N input rate, with more N removed in products, the amount of N that be sustained in the system under irrigation is reduced (run down to a lower sustainable steady state). With greater N removal, for a fixed N input,  there is also less N per ha that can sustainably be released to the environment, as seen in (c).

The lower amount of N available and sequestered, reduces the amount of C that can be sequestered in the (eg soil) organic matter.

Because irrigation increases the yield of products per ha, (a) and inevitably decreases the release of N per ha (c), it substantially reduces the footprint (kg N released to the environment per unit products)  at the same N input rates, as seen in (e). The methane intensity (kg CH4/kg N_products) alters little with irrigation (as both numerator and denominator are linked to the same thing, yield of products per ha.


Interaction with animal class

As described under ‘dairy vs meat’ , a far greater proportion of the N in the diet of dairy animals (‘L’) is removed in products, compared to that removed in a meat system (‘M’). This is true per animal, and in turn under a common grazing regime, the same applies per hectare. Hence the substantially greater Nitrogen Use Efficiency (kg N in products/kg N input), in a dairy ‘L’, than in a meat ‘M’, system, as seen again below in (g).

The outcome of the interaction between irrigation and water inputs is different for ‘dairy’ than for ‘meat’ in the context also of C sequestration, see (b) above.

We have described how a stimulus to yield, due to irrigation, can induce a run down in the amount of N in the system, and so irrigation can induce sustained N deficiency, notably at low-intermediate N input rates. Because ‘dairy’ animals remove a greater amount of N per ha, than  ‘meat’ animals, the tendency to inducing  N deficiency is greater when the harvesting animals are ‘dairy’ rather than ‘meat’.

Hence, at the same low-intermediate N input rates, see Figure (b) above, less C can sustainably be sequestered under ‘dairy’ than under ‘meat’, for a given level of water input.



Emissions intensity graphs corresponding to (a) to (d) above:









and corresponding ‘nitrogen use efficiency’ graph (below) :











Irrigation : time course of changes in C and N balance following ‘intensification’

see ‘C sequestration’.