Considerations of a field-scale soil carbon budget for furrow irrigation

There is a general lack of information on the effects of irrigation on soil carbon (C) sequestration in (semi)arid regions. For that purpose we present results of the sediment and C budget of a 30 ha furrow-irrigated corn field in the Central Valley in California. This field was monitored to assess the effects of minimum tillage versus standard tillage on soil C sequestration and greenhouse gas emissions. Water samples of two irrigation events in July and August 2004, were collected and analyzed for suspended sediment, dissolved organic C (DOC) and N (DON), total C and N. Field and soil water budgets were estimated from meteorological data, flow measurements of applied irrigation and runoff water, and neutron-probe soil water measurements. Tail waters contained less sediment but more organic C than irrigation waters, due to particle settlement and enrichment in organic matter. Tillage treatment had no significant effect on composition of water or sediment. Furrow irrigation resulted in a net field input of 700 kg sediment ha−1, 21.4 kg C ha−1, and 7.7 kg N ha−1. The added C by the sedimentation accounted for about two-thirds of the total C increase. The corresponding soil C increase associated with these two irrigation events was about 20% of reported yearly C sequestration rates in long-term soil C sequestration experiments. Our experiments showed the importance of time scale in C budgeting for intensively irrigated agroecosystems, where fast dynamics and large variability of inputs are common.