Gross nitrogen mineralization in pulsecrop rotati

Authors: Angela BedardHaughn LouisPierre Comeau Amy Sangster

Publish Date: 2013/02/12

Volume: 95, Issue: 2, Pages: 159-174

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Abstract

Pulse crops represent an everincreasing proportion of cropping systems in the Northern Great Plains Previous studies have noted apparent benefits associated with pulse crop production that extend beyond the reduced need for N fertilizer in the year of production these benefits have been attributed to the quality of pulse residues and their effects on N dynamics in subsequent years This study used isotope dilution techniques to quantify the Ncycling effects of pulse crops in the rotation Gross N mineralization was measured over three growing seasons at two Agriculture and AgriFood Canada research sites in Saskatchewan Canada Scott four rotations one with pulse crop and Swift Current three rotations one with pulse crop Gross nitrification and the relative contribution of nitrification vs denitrification to N2O emissions were also measured Across all dates and rotations the average gross mineralization rate at Scott was 20 ± 40 mg NH4 +N kg−1 soil d−1 and at Swift Current was 14 ± 39 mg NH4 +N kg−1 soil d−1 At both sites rates were highly variable across the growing season but tended to be higher at anthesis than either preseeding or postharvest The only significant difference among rotations was at Swift Current where the fertilized continuous wheat rotation had the highest gross mineralization rates rotation average 23 mg NH4 +N kg−1 soil d−1 The lack of difference among most rotations was particularly notable given the differences in residue quantity among the crops Ultimately the lower quantity of residues produced by pulse crops appears to be offset by their higher qualityCropping systems in the Northern Great Plains have changed considerably since the beginning of agricultural production Management modifications in these systems can have a profound effect on agroecosystem sustainability due to the scale of production Saskatchewan alone has over 26 million ha of cropland in agricultural production Statistics Canada 2006 In recent years there has been an unprecedented shift to pulse and oilseed crops in dryland cropping regions Miller and Holmes 2005 therefore new interest has developed regarding the effects of these crops on soil processes such as N cycling and the effect of pulse crops on subsequent crops in rotation Miller et al 2006 Tanaka et al 2007 Malhi et al 2009 Depending on conditions pulse crops such as field pea Pisum sativum can biologically fix up to 90  of their total plant N from the atmosphere Walley et al 2007 In contrast canola N requirements are commonly met through the use of synthetic N fertilizers at recommended application rates as high as 120 kg N ha−1 for maximum seed yield Gan et al 2007 The reduced fertilizer N input for the pulse production year might not be the only benefit received from including pulse crops in rotation Given that most of the fixed N is removed with the highprotein grain yield benefits to crops following field pea have typically been attributed to N mineralized from field pea residue Stevenson and van Kessel 1996 Beckie et al 1997 Raun and Johnson 1999 Johnston et al 2005 Miller and Holmes 2005 However these N benefits are highly variable from year to year and are not consistent across all pulse crops Walley et al 2007Pulse N contribution is most commonly assessed by measuring soil N pools rather than processes For example Soon and Arshad 2004 reported spring available N concentrations were higher in rotations following legumes than cereal crops Although net mineralization provides an index of plant available N it does little to explain the total amount of N cycling between organic matter and soil inorganic N Robinson 2001 Gross rate measurements on the other hand provide estimates of the total release of mineral N from a given pool Gross processes of mineralization ammonification nitrification and consumption occur simultaneously in the soil and their relative magnitudes will determine whether there is a net release of N into the soil Recous et al 1999 Murphy et al 2003 However even if net N accumulation is low there will likely be plantavailable N actively being produced throughout the season Schimel and Bennett 2004 Estimates of gross N fluxes may help in predicting N availability in the soil in particular when these measures of inorganic N production are repeated throughout the growing season providing a better idea of when maximum mineralization occurs Despite the potential to improve fertilizer timing these methods are not commonly used on the Great Plains A comprehensive review of gross N cycling studies included just three studies on gross N mineralization in semiarid agricultural landscapes and just one on gross nitrification Booth et al 2005Mineralization and nitrification in addition to soil water content and temperature also influence the rate of nitrous oxide N2O emissions in agricultural soils Grant et al 2004 Nitrous oxide is a potent greenhouse gas with a 100year global warming potential 298 times that of carbon dioxide IPCC 2007 In Canada N2O emissions resulting from transformations of mineral N in agricultural soils make up the largest proportion of total N2O emissions Rochette and McGinn 2008 However N2O emissions from pulse crops may be lower than their fertilized counterparts Lemke et al 2002 likely due to decreased losses of N2O from fertilizer applications Pulses may also affect the other factors contributing to emissions such as the quantity and quality of substrate residue supply for mineralization and nitrification of organic NAlthough N2O emissions have been attributed to several processes including nitrification denitrification coupled nitrification–denitrification chemodenitrification and nitrifier denitrification Wrage et al 2001 the two main processes believed to contribute to soil surface N2O flux in arable agriculture are denitrification and autotrophic nitrification Pennock 2006 There are many reports of N2O emissions in the semiarid Northern Great Plains Corre et al 1996 Malhi et al 2006 Yates et al 2006 However there are few field experiments that report on the relative contributions of nitrification and denitrification to total N2O emissions BedardHaughn et al 2006 Ma et al 2008 Stable isotope techniques have enabled differentiation and quantification of N2O produced during denitrification and nitrification Baggs 2008A greater understanding of the effect of pulse crops namely field pea and lentil on N mineralization will help better delineate potential N benefits during the growing season and to succeeding crops This study took advantage of established research plots in the Northern Great Plains that included cropping rotations both with and without pulse crops This should allow a broader perspective on the longerterm effect of pulse crops in the rotation Gan et al 2010 The primary objective of this paper was to evaluate the effect of pulse crops in the rotation on gross mineralization rates including the variability of these rates across the growing season A secondary objective was to quantify the effect of pulse crops in the rotation on gross nitrification including the relative contribution of nitrification related pathways to total N2O emissions

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