Effects of irrigation and nitrogen addition on the components of net ecosystem carbon balance in New Zealand grazed grasslands.

Abstract

There is growing evidence that continued emissions of greenhouse gases from anthropogenic activities associated with climate warming are responsible for widespread, severe and irreversible impacts leading to insecurity of food supply to support the needs of increasing population growth. Improved use of agricultural land and management practices can result in increased soil carbon sequestration and are critically important to maintain productivity and mitigate the effects of anthropogenic carbon dioxide emissions. Globally, there is a trend towards intensification of grassland systems to increase agricultural productivity. While this ensures increased pasture growth, and supports higher number of grazing animals per unit area, the impacts on ecosystem carbon dynamics remain largely unknown. This is of particular importance in New Zealand, where grasslands occupy large areas and where there is rapid and widespread expansion of intensification using irrigation and nitrogen fertilisers to convert traditional extensively grazed sheep and beef pastoral systems to dairy farming. The research in this thesis investigates the following question: Will the conversion to intensive dairy farm management result in soil carbon sequestration in New Zealand managed grasslands? The ability to quantify and predict the impacts of changing management practices on soil carbon stocks and their potential feedbacks on atmospheric CO2 concentration largely depends on our ability to understand the mechanisms regulating changes in the components of the net ecosystem carbon balance: gross photosynthesis, ecosystem respiration, and the autotrophic and heterotrophic components of soil respiration. To investigate the impacts of management practices on these components, I designed a series of laboratory and field experiments on shallow, stony soils in Canterbury, New Zealand, that are characteristic of sites where widespread conversion to dairy farming is occurring. The experimental treatments allowed measurement and modelling of the effects of irrigation, addition of nitrogen fertiliser and intensive grazing management. To account for variations in above-ground biomass through seasons and grazing cycles, I developed a phytomass index and used this in models to interpret changes in the components of carbon balance. Changes in carbon dynamics under irrigation and nitrogen addition were dominated by above-ground processes and consistently resulted in increased net ecosystem productivity. Measuring the effects of the treatments during the first year after a simulated conversion of dryland to dairy farming, I showed net losses of carbon from the ecosystem, ranging from 284 gC m-2 y-1 to 540 gC m-2 y-1 across combinations of irrigation and nitrogen addition treatments, although uncertainties were large, so that differences were not significant. From the findings, I hypothesise that losses of carbon from the grassland soil at this study site will continue to exceed ecosystem carbon inputs, despite increased above-ground productivity. To explore this further, I adopted and improved techniques using stable isotopes of carbon to measure rates of soil organic matter decomposition in the field, while avoiding disturbance to the soil environment. The findings showed no effects of irrigation and addition of nitrogen fertiliser on the rates of soil organic matter decomposition. I used indices of soil physical protection of organic matter to interpret differences in the rates of decomposition and provided direct evidence to support the emerging theory that soil organic matter decomposition is predominantly regulated by microbial access to the carbon substrate. From earlier studies on long-term changes in soil carbon stocks and the new insights from my research, I conclude that there is no evidence that conversion to intensive dairy farming on shallow, stony soils in New Zealand leads to beneficial effects for soil carbon stocks. The effects are likely to depend on soil type and management practices that determine the level of organic matter protection and its stability and more research is needed to identify mechanistic linkages between soil organic matter dynamics and soil structure. The new insights from my findings have important implications for our ability to model and predict future changes in carbon stocks for grasslands globally, in relation to changing climate and management practices.