Irrigation effects on soil organic carbon under a ryegrass-white clover pasture on a Lismore stony silt loam soil : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Long-term summer irrigation of temperate managed pastures has been reported to either increase or decrease soil organic carbon (SOC) stocks when compared with dryland systems. Understanding the short-term effects of irrigation on the assimilation, partitioning and storage of carbon (C) within the plant-soil system is important in order to identify key mechanisms that explain the observed differences in SOC responses to irrigation. Two continuous ¹³CO₂ pulse labelling experiments were performed to ¹³C-labelled mesocosms established with ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) mixed pasture using a Lismore stony silt loam soil (Pallic Firm Brown soil). This PhD study has demonstrated that summer irrigation applied to increase above-ground pasture productivity in managed ryegrass-white clover pastures had no demonstrable effects on the storage and loss of new photosynthate C partitioned into the soil over an annual production cycle when compared with dryland conditions. Therefore, based on these results it can be concluded that under the conditions of this research, summer irrigation had a neutral effect on the formation and short-term (< 1 year) storage of newly formed SOC when compared with dryland conditions. However, the study of soil C components or pools (e.g. rhizosphere soil and soil particle size fractions) demonstrated that irrigation affected the spatial and temporal partitioning of root derived-C in the soil, when compared with a dryland pasture system, by reducing the accumulation of new photosynthate C in the rhizosphere soil while increasing the accumulation of this new C in the fine POM (53–250 µm) and clay (< 5 µm) size fractions of the non-rhizosphere soil. This re-distribution of root derived-C among the soil C components occurred relatively early in the irrigated pasture over the summer and autumn seasons, while in the dryland pasture the re-distribution occurred over the following autumn, winter and spring seasons. In addition, the results showed that with irrigation the root system became smaller and shallower, with the pasture biomass allocation to above-ground plant components being favoured. This effect persisted over an annual pasture growth cycle, especially during the autumn and spring seasons following the cessation of summer irrigation. These findings may have implications for the longer-term storage of SOC in dryland relative to irrigated pasture systems that were not tested in this study. Further research is needed to improve our understanding on the mechanisms driving the different responses of SOC to irrigation with a particular focus on: (i) determining the causes for the higher root turnover under irrigated pastures and, (ii) evaluating how irrigation interacts with other pasture management practices (e.g. fertiliser, grazing) to affect the formation, function and storage (> 1 year) of SOC, including that associated with POM and MAOM in soils.