Green-manure crops as a source of nitrogen in cropping systems

The focus of this thesis is on tailoring the release of N from green manure that has been incorporated into the soil, with the aim of improving the efficiency with which green-manure N is used in cropping systems. The mineralization of N from ploughed under green-manure crops was studied as well as mineralization of N from harvested green-manure material of different species and plant fractions during decomposition in soil. The influence of the green-manure supply on the yield and N content of spring wheat (Triticum aestivum L.) as determined. Field studies and pot experiments under both greenhouse and outdoor conditions were conducted. In field experiments the grain yield of spring wheat was up to 2500 kg ha-1 larger after whole-season Persian clover (Trifolium resupinatum L.) green-manure than after oats (Avena sativa L.) but the effect on wheat protein content was less pronounced. Incorporation of legume material from species in their pre-flowering stage made a large short term contribution of available N to the succeding crop. After 8000 kg ha-1 dry matter of white clover (T. repens L.) and yellow sweetclover (Melilotus officinalis (Lam.) herbage had been incorporated, 142 and 191 kg N ha-1, respectively, were mineralized. Both lower N mineralization rates and smaller amounts of N released, 44 and 80 kg N ha-1, respectively, were obtained when red clover (T. pratense L.) and perennial ryegrass (Lolium perenne L.) were applied. When harvested herbage was ensiled the amount of mineralized N was reduced by an average of 17% as compared with dried herbage. On the other hand, silage was suggested to contribute more to the accumulation of organic N in soil. It was concluded that the leaf proportion of total herbage dry matter of the green-manure crop has a major influence on subsequent mineralization of N. On average, more than four times as much N was released from red clover and sweetclover leaves compared with amounts released from the stem materials. After 8,5 months of soil decomposition, on average 55% of the applied N in different dried plant fractions of red clover and yellow sweetclover was recovered as organic N in soil.