The Effects of substrate, temperature and soil fertility on respiration and N2O production in pastoral soils

Soil respiration (Rs) and N₂O emissions from pastoral ecosystems are responsible for a substantial portion of global greenhouse gas budget. The soil processes responsible for RS and N₂O emissions are sensitive to soil temperature (TS). However, there are many points which are uncertain in this temperature sensitivity of soil processes, because of the complexity of the mechanisms controlling the processes. The temperature sensitivity is defined as a proportional increase of the rate of soil process or activity per a unit change of soil temperature. An important factor controlling the temperature response of the soil processes is substrate availability. Hence the objectives of this research were (1) to quantify the interaction between temperature and soil substrate availability on soil microbial activity in the absence of plant substrate inputs, (2) to determine the temperature sensitivity of respiration sourced from root-derived C (RRD) and soil organic matter decomposition (ROM) in two pasture soils of contrasting nutrient status, and (3) to investigate the effects of a urine deposition events on RS and N₂O fluxes. The first experiment measured the changes in soil microbial respiration (RM) without plants present at 3°, 9°, and 24°C. At 9° and 24°C, RM was significantly reduced within 2 days while RM remained constant for 14 days at 3°C. The decrease in RM at higher TS was caused by substrate depletion but the substrate depletion was not indicated by the soil’s water soluble and hot-water soluble C. The first experiment showed that at higher TS, soil microbes could access soil C that was not accessible to soil microbes at lower TS. The second experiment focused on the temperature sensitivity of RS with plants present in two soils with contrasting nutrient status (fertility). The components of RS; RRD and ROM were separatedly measured using a natural ¹³C abundance technique. The results suggested the temperature sensitivity of ROM was significantly reduced in the low fertility soil only when plants were actively growing, while the temperature sensitivity of RRD was unaffected by soil nutrient status. Finally the third experiment investigated the changes in soil N₂O emissions and RS following a urine deposition event on a pasture soil at various TS (11°, 19°, and 23°C) with or without plants present. Soil moisture was increased from 50% to 70% water-filled pore space at 21 days following the urine deposition event. Soil-N contributed to soil N₂O emissions only at the early phase of the experiment, especially at higher TS, and the contribution was lesser when plants were present. The presence of plants increased N₂O flux particularly when soil moisture contents were high, and when TS was > 19°C. Urine application primed soil C and increased the rate of RS. The magnitude of urine-induced priming of soil C was relatively larger at lower TS, and was larger when plants were absent based on the estimation from cumulative RS. Based on the results obtained using a natural ¹³C abundance, when plants were present, urine application increased the contribution of ROM to RS particularly at 19°C. This study indicates that the responses of soil N₂O fluxes and RS to temperature changes are markedly affected by plant presence and soil nutrient status. Both urine addition and plant activity primed RS but the magnitude of the priming effect was influenced by other factors (e.g. TS). To accurately predict the global warming feedback of belowground soil processes, the factors affecting the aboveground plant activity (e.g. soil nutrient status) have to be taken into account.