Effects of irrigation and nitrogen fertilizer on yield, carbon inputs from above ground harvest residues and soil organic carbon contents of a sandy soil in Germany B. T ROST 1 , F. E LLMER 2 , M. B AUMECKER 3 , A. MEYER -A URICH 1 , A. P ROCHNOW 1,4 & K. D RASTIG 1 1 Leibniz-Institute for Agricultural Engineering Potsdam-Bornim, Max-Eyth-Allee 100, 14469 Potsdam, Germany, 2 Division of Agronomy and Crop Science, Humboldt-University of Berlin, Faculty of Agriculture and Horticulture, Albrecht-Thaer-Weg 5, 14195 Berlin, Germany, 3 Humboldt-University of Berlin, Faculty of Agriculture and Horticulture, Field Study and Research Station, Dorfstrasse 9, 14974 Thyrow, Germany, and 4 Chair Utilization Strategies for Bioresources, Humboldt-University of Berlin, Faculty of Agriculture and Horticulture, Hinter der Reinhardtstr. 8-18, 10115, Berlin Germany Abstract To better understand the complex interactions between irrigation and nitrogen fertilizer application on soil organic carbon content, the results from long-term field experiments over a period of 40 years were analysed. The combined effect of irrigation and nitrogen fertilizer rates on crop yields, carbon input by above ground harvest residues and soil organic carbon content has been investigated at a site on a sandy soil in northeast Germany. Combined with nitrogen fertilizer application, irrigation has frequently had a significantly positive effect on crop yield and carbon inputs from above ground harvest residues. However, enhanced carbon inputs to the soil under irrigation did not lead to significantly greater soil organic carbon contents. As the combination of irrigation and nitrogen also improved microbial decomposition by changing of above ground harvest residues C/N ratio and soil moisture, the effect of an additional input of carbon from above ground harvest residues was nullified. Keywords: Soil organic carbon, irrigation, nitrogen fertiliser application Introduction Soil organic carbon (SOC) contributes to various services that benefit the farmer, such as soil fertility, together with society, for example carbon sequestration and mitigation of global warming (Schulz, 1997, 2004; Lal, 2004). The type and intensity of agronomic operations in crop production have significant effects on the SOC content. While it is well known that inadequate fertilizer application and intensive tillage results in decreasing contents of SOC (Lal, 2002; Baker et al., 2007), the effects of other agronomic operations, such as irrigation, on SOC content have not been investigated in such detail. Irrigation is essential for crop production in many regions of the world (Howell, 2000; Fallahzade & Hajabbasi, 2012). Despite its relative importance, there are not many studies on the influence of irrigation on SOC contents (for overview, see Trost et al., 2013). It is generally accepted that irrigation enhances biomass production and the amounts of above ground harvest residues, and as a consequence, this results in larger carbon inputs into the soil (Watson et al., 2000; Rold an et al., 2005; Entry et al., 2008). Concomitantly, increased soil moisture may improve conditions for microorganisms. Greater microbial activity could result in enhanced decomposition of soil organic matter with the effect of increased emissions of CO 2 from soil (Van Gestel et al., 1993; Gillabel et al., 2007; De Bona et al., 2008; Jabro et al., 2008). Studies directly comparing SOC contents on irrigated and nonirrigated fields are rare. Long-term investigations, especially those on arid sites, show that in many cases, greater SOC contents were observed under irrigation than under nonirrigated conditions (Entry et al., 2004; Gillabel et al., 2007; Denef et al., 2008; Li et al., 2009; Su et al., 2010). However, irrigation did not affect SOC content on sites containing more SOC (Bordovsky et al., 1999; Dersch & B€ ohm, 2001; De Bona et al., 2008). It was also found that irrigation in combination with intensive cultivation and removal of harvest residues resulted in a decrease in SOC Correspondence: B. Trost. E-mail: btrost@atb-potsdam.de Received March 2013; accepted after revision April 2014 © 2014 British Society of Soil Science 209 Soil Use and Management, June 2014, 30, 209–218 doi: 10.1111/sum.12123 Soil Use and Management