Dynamics and application of hot water extractable carbon and nitrogen in New Zealand pastoral systems : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Agricultural Science at Massey University, Manawatū, New Zealand
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Date
2024
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Massey University
Figures 3.6 and 3.7 are reused with permission.
Figures 3.6 and 3.7 are reused with permission.
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Abstract
Changes in soil carbon (C) and nitrogen (N) stocks under agricultural land have the potential to reduce greenhouse gas emissions and improve soil health. The health of soils is dependent on the organic matter cycle, biological community and its functions and activity. While current tests used for these parameters are often expensive and time-consuming, hot water extractable C (HWEC) and hot water extractable N (HWEN) measures have been promoted as sensitive and informative tests of labile and microbial C and N. The consensus of studies to date, is that factors such as soil type heavily influence the HWEC and HWEN measures. However, information on how other characteristics typical of New Zealand farms, such as highly variable topography, different phosphorus (P) fertiliser history, grazing management and stocking densities and crop establishment affect HWEC and HWEN measures is lacking. Furthermore, the responsiveness of these tests to land management changes within a farm system has yet to be fully investigated. The first study investigated changes in soil HWEC and HWEN pools with varying long term P fertiliser history, slope, aspect and sheep stocking rates. Significant differences in HWEC were not observed between high P fertiliser farmlets and low P fertiliser farmlets, only between the low P fertiliser farmlet and the farmlet which had not received P fertiliser since 1980. Significantly lower HWEC and HWEN concentrations were observed on high, erosion prone slopes when compared to medium and low slopes. The prediction of HWEC using soil physicochemical factors, taking into account P fertiliser history and slope, has advanced our understanding of factors influencing the HWEC fraction. These factors were organic matter, effective CEC, microporosity and sulfate sulfur (S). The second study investigated the impact of adaptive, high instantaneous (number of animals per unit area at a specific point in time) stocking density grazing on HWEC and HWEN. This study found adaptive grazing did not increase either the HWEC or HWEN pool. The final study investigated the impact of summer crop establishment using herbicide and power harrowing and herbicide and no-till methods on HWEC and HWEN. Results suggest there was no detrimental effects on HWEC and HWEN associated with no-till, and power-harrowing yielded a similar result after a period of soil recovery. Overall, HWEC and HWEN proved to be highly indicative of the total C and N. None of the changes investigated in this study had major effects on the quantity of total C and N pools present as HWEC or HWEN.