Nitrogen release from field pea residues and soil inorganic N in a pea wheat crop rotation in northwestern Canada

Lupwayi, N. Z. and Soon, Y. K. 2009. Nitrogen release from field pea residues and soil inorganic N in a pea wheat crop rotation in northwestern Canada. Can. J. Plant Sci. 89: 239-246. Pea (Pisum sativum L.) varieties can differ in morphology, N2 fixation and straw N content. A study was conducted over 3 site-years to evaluate the influence of pea variety and inoculation with Rhizobium on N release from decomposing pea residues. The litterbag technique was used to measure N release over a 52-wk period starting from the time of pea harvest in one season through part of the following season when wheat was grown. Experimental treatments comprised factorial combinations of three pea varieties and either inoculation with 5 kg ha-1of a granular inoculant or none, arranged in a randomized complete block design. Neither pea variety nor inoculation affected amounts or patterns of N released. Patterns of N release over time showed mostly net N mineralization in two of 3 site-years, and some net N immobilization in one site-year. The percentages (up to 19 to 24% over time) and amounts (up to 2.3 to 7.5 kg N ha-1) of N released were low, probably due to the combination of low N concentrations (mostlyB1%) in the residues and below-normal rainfall in all 3 site-years. Soil NO3-N and NH4-N (0- to 80-cm depth) in the fall after pea harvest (20 to 39 and 27 to 55 kg N ha-1, respectively) and in spring before wheat seeding (23 to 51 and 16 to 40 kg N ha-1, respectively) were not affected by pea variety or inoculation. However, soil NO3-N was mostly higher after peas than after barley (the control). There is need to measure patterns of N release over several subsequent crops to check if more N is released in the long term.