Effects of long-term annual inputs of straw and organic manure on plant N uptake and soil N fluxes

This study investigated the effects of long-term annual inputs of animal manure and straw on the rate of gross nitrogen (N) mineralization-immobilization turnover (MIT), net N mineralization and potential nitrification, and examined how these N transformation rates affect plant N availability. The experiment was conducted during May-June 2001 in long-term field experiments in Askov, Denmark, where organic manure and barley straw had been applied annually for 11 and 20 years prior to the year 2000, respectively. Thus, any differences could be attributed to residual effects from the previous years of application. Inputs of straw and organic manure to soil increased soil organic matter (SOM)-N content in soil in the order: without straw, without manure < without straw, with manure < with straw, without manure < with straw, with manure. The inputs did not change net N mineralization in the soil. There was a distinct but non-significant trend towards higher gross N mineralization with increasing SOM-N. Gross N immobilization was enhanced by straw inputs and to a lesser extent by organic manure inputs, while potential nitrification was enhanced by both amendments. The results show that long-term annual inputs of straw and organic manure can increase MIT rate and potential nitrification rate without influencing net N mineralization rate. MIT and potential nitrification explained 23-31% of the variation in plant N uptake, while net N mineralization rate only explained 1%. Plant N uptake therefore seems to be more influenced by MIT rate and potential nitrification rate than by net mineralization rate, presumably because mineral N in the transition between gross N mineralization and gross N immobilization is available for assimilation by plants.