[Truncated] More efficient management of nitrogen (N) in agricultural soils is vital to maximise food supply and minimise losses of N to the environment. Nitrification is a key pathway of detrimental N loss, as nitrate and gaseous nitrous oxide are produced. In semi-arid soils, N cycling and nitrification is not well understood during summer fallow, an important period for N loss, as most research has instead focussed on N fertiliser management during the growing season. In order to better understand and manage N cycling in cropped semi-arid soils, this thesis investigated factors contributing to risk of N loss, as well as possible solutions to decrease the risk of loss. The close link between soil N and carbon (C) cycling suggested that solutions might be found through management of soil organic matter. Soil was used from a long-term field site in the northern grainbelt of Western Australia with a range of crop residue and tillage treatments (no tillage; no tillage with burnt stubble; tillage; tillage plus additional crop residue inputs; and tillage plus crop residues run-down) that altered soil organic matter content since 2003, allowing examination of N transformation pathways without confounding effects of differing soil types or climate.
Firstly, steady-state N transformations and risk of N loss (defined as gross nitrification: immobilisation ratio) were examined, in response to a range of soil temperatures, root exudate C and field treatment (tilled soil and tilled soil plus crop residues), using 15N isotopic pool dilution and turnover of 14C-labelled substrates. Tilled soil plus crop residues had 76% more total C than tilled soil. Root exudates were effective at decreasing risk of N loss by stimulating microbial N immobilisation over nitrification. In comparison, management of N loss through additional crop residue inputs was unlikely to be effective, as increased soil organic matter enhanced the supply of both C and N substrates and N cycling overall. At temperatures above 30 °C, net N mineralisation was associated with decreased microbial C use efficiency, likely contributing to increases in inorganic N pools during summer fallow.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 2015|