Short-term effects of graminaceous cover crops on autumn soil mineral nitrogen cycling in western lower Fraser Valley soils
Creators: name: Nafuma, Leonard Simiyu
Publisher: name: University of British Columbia
Year: 2009

description: Proper cover crop management practices in autumn can minimize N0₃⁻-N leaching. Three experiments to study the effect of cover crop management on autumn soil mineral N conservation were conducted in the 1991-92, 1992-93 and 1993-94 winter seasons on a silty clay loam Rego Humic Gleysol in the western Lower Fraser Valley, British Columbia, Canada. The study tested short-term effects of planting date, autumn soil mineral N content and type of cover crop on biomass production and N uptake, residual soil mineral N (0-60 cm layer), plant composition of various N fractions of autumn-planted spring species at winter-kill, retention of accumulated N by autumn-planted spring species after winter-kill, and the C/N ratio of cover crops. Treatments involved two planting dates (late August and September), two simulated autumn residual mineral N levels (0 and 100 kg N ha⁻¹) and types of cover crops. In the first two seasons, the cover crop treatments were spring barley (Hordeum vulgare L.) and winter rye (Secale cereale L.) plus fallow for comparison purposes. In the third season, planting date was omitted and six cover crop treatments tested were spring barley, spring wheat (Triticum aestivum L.), spring oat (Avena sativa L.), winter rye and annual ryegrass (Lolium multiflorum Lam.) including fallow. Planting crops in August as compared to a month later increased biomass production by 56 to 135% and N uptake by 38 to 93% before winter leaching period. Large N uptake by cover crops that were planted in August was generally accompanied by significant reduction in soil mineral N (0-60 cm) from August to November. August-planted spring species N at winter-kill was largely composed of the protein fraction (insoluble and soluble) which increased with N supply in autumn when the initial mineral N contents in 0-60 cm layer of soil were suboptimal (&lt; 100 kg N ha⁻¹) but was not affected when soil mineral N content was 200 kg N ha⁻¹ and more or when the cover crops were planted in September. There were indications that August-planted spring species can retain some of the soluble protein N fraction in the winter-killed residues during winter. Maximum plant N0₃⁻-N content represented about 15% (~ 20 kg N ha⁻¹) of the total N in the plant when cover crops were planted in August and autumn soil mineral N content (0-60 cm) was about 200 kg N ha⁻¹ or more. The proportion of NH₄⁺-N averaged only 3%. Spring species can be included in winter cropping systems in western Lower Fraser Valley. Spring species that were planted in August and winter-killed in late autumn showed greater potential to retain the N accumulated before winter-kill compared to the cover crops that were planted a month later. August-planted spring species increased soil mineral N (by 40 to 76%) in the 0-60 cm layer in spring relative to fallow plots while September-planted crops had little effect. It appears, spring species can play a significant role in autumn mineral N conservation by accumulating large amounts of autumn soil mineral N before winter leaching period, retaining it in winter-killed residues until spring and releasing the N in plant available form through decomposition and mineralization.