NITROGEN ACQUISITION BY PEA AND BARLEY AND THE EFFECT OF THEIR CROP RESIDUES ON AVAILABLE NITROGEN FOR SUBSEQUENT CROPS

Nitrogen acquisition by field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) grown on a sandy loam soil and availability of N in three subsequent sequences of a cropping system were studied in an outdoor pot experiment. The effect of crop residues on the N availabi lity was evaluated using N-15-labelled residues. Field pea fixed 75% o f its N requirement and the N-2 fixation almost balanced the N removed with the seeds. The barley crop recovered 80% of the N-15-labelled fe rtilizer N supplied and the N in the barley grain corresponded to 80% of the fertilizer N taken up by the crop. The uptake of soil-derived N by a test crop (N catch crop) of white mustard (Sinapis alba L.) grow n in the autumn was higher after pea than after barley. The N uptake i n the test crop was reduced by 27% and 34% after pea and barley residu e incorporation, respectively, probably due to N immobilization. The d ry matter production and total N uptake of a spring barley crop follow ing pea or barley, with a period of unplanted soil in the autumn/winte r, were significantly higher after pea than after barley. The barley c rop following pea and barley recovered 11% of the pea and 8% of the ba rley residue N. The pea and barley residue N recovered constituted onl y 2.5% and <1%, respectively, of total N in the N-fertilized barley. T he total N uptake in a test crop of mustard grown in the second autumn following pea and barley cultivation was not significantly influenced by pre-precrop and residue treatment. In the short term, the incorpor ation of crop residues was not important in terms of contributing N to the subsequent crop compared to soil and fertilizer N sources, but re sidues improved the conservation of soil N in the autumn. In the long- term, crop residues are an important factor in maintaining soil fertil ity and supplying plant-available N via mineralization.