Temperature and Moisture Sensitivity of Soil Microbial Respiration in Adjacent Irrigated and Non-Irrigated Soils

Irrigation is a commonly used management practice that is crucial for increasing plant growth and production, especially in areas prone to drought such as the Canterbury region of the South Island, New Zealand. Historically, irrigation was thought to increase soil carbon content due to increased production, however recent studies have shown that irrigation causes a loss of soil carbon (C). One possible mechanism for the C loss is an increase in soil microbial respiration under irrigation. The added soil moisture under irrigation releases microbial moisture limitations and enables soil microbes to access more C, therefore increasing respiration and decreasing soil C content. Soil microbial respiration also fluctuates seasonally. Irrigation changes the inherent seasonal effect by increasing soil moisture content during the hottest part of the year, therefore increasing soil microbial respiration rates. In this thesis soil samples were collected from 13 paired irrigated and non-irrigated sites in Canterbury and two sites at Rangiriri in the Waikato region of the North Island. The sites in Canterbury were sampled once while the sites at Rangiriri were used for a seasonal analysis and were sampled twice. Soil samples were wet to five different moisture contents and incubated on a temperature gradient block for five hours. MMRT curves were then fitted to the respiration data obtained from incubation and the temperature optima (Topt), temperature inflection point (Tinf) and change in heat capacity (ΔCp‡) were calculated. The absolute respiration rates at 10°C (R10) and 20°C (R20) were also calculated. Irrigation had a significant effect on soil microbial respiration in the Canterbury soils but not the Rangiriri soils. In Canterbury, the Topt and Tinf were higher in the irrigated soils by 8.8°C and 7.6°C respectively while the R10 and R20 were both nearly 50% higher in the non-irrigated soils and all differences were statistically significant. There was no difference between treatments in any of the parameters measured in the Rangiriri soils. The difference in temperature sensitivity and absolute respiration rate in the Canterbury soils was thought to be due to the soil microbes under irrigation decomposing less readily available soil C which has a higher temperature sensitivity and leads to a reduced respiration rate. Another possible explanation for the differences in temperature sensitivity and absolute respiration rate was that there had been a shift in soil microbial community structure between the two treatments and this should be further investigated