Biomass Production and Nitrogen Accumulation by Hairy Vetch–Cereal Rye Mixtures: A Meta‐Analysis

Abstract

Agroecosystem services from cover crop mixtures are linked to aboveground biomass and total N content (kg ha-1). Reported values in the literature, however, vary for aboveground biomass and total N content of cover crop mixtures compared with monocultures. We conducted a meta-analysis using results from 55 site-years from 21 studies conducted in the United States to examine biomass and N content of hairy vetch (Vicia villosa Roth)-cereal rye (Secale cereale L.) mixtures compared with respective monocultures. Overall, hairy vetch-cereal rye mixtures produced 63 and 21% more biomass compared with hairy vetch and cereal rye monocultures, respectively. The N content of hairy vetch-cereal rye mixtures was 150% greater than that of cereal rye monocultures. When the proportion of hairy vetch seeds (by weight) exceeded 46% of the mixture, the mixtures accumulated equivalent or more N than the greatest yielding monocultures (usually hairy vetch). Compared with monocultures, a more consistent positive response of mixtures on biomass and N content was found on coarse-textured soils and following corn (Zea mays L.) rather than soybean [Glycine max (L.) Merr.] harvest. With increasing growing degree days (GDD), the biomass and N content of mixtures decreased relative to hairy vetch monocultures but increased relative to cereal rye monocultures, suggesting better performance of hairy vetch at higher GDD. We conclude that hairy vetch-cereal rye mixtures can produce equivalent or more biomass than both monocultures and accumulate as much N as hairy vetch, and that the relative productivity of mixtures depends on soil type, previous crop, seeding proportion, and GDD.

Cover Crops Reduce Nitrate Leaching in Agroecosystems:A Global Meta‐Analysis

“…1a). Besides NO 3 − leaching reductions, the intermediate C/N ratio (25–30:1) in the residues from nonlegume–legume cover crop mixtures results in a more balanced N mineralization and immobilization turnover and improves N synchrony as compared with both nonleguminous and leguminous cover crops (Ranells and Wagger, 1997; Rosecrance et al, 2000; Poffenbarger et al, 2015; Thapa et al, 2018). Therefore, if the goal is to effectively reduce NO 3 − leaching and reduce N fertilizer requirements of the subsequent cash crop through cover crop N credits, we suggest cover crop mixtures composed of both nonleguminous (for N scavenging) and leguminous (for N supply) components.…”

“…For moderator analysis, a separate ln( R ) was calculated using each moderator variables as a sole covariate in the multilevel mixed effect meta‐analytic model described above; robust SEs were estimated following cluster‐based robust variance estimation. To protect against experiment‐wise Type I errors, we calculated 99% CI around ln( R ) for each moderator variable (Thapa et al, 2018). The mean effect sizes for each moderator variable were considered significant ( p < 0.01) only if the 99% CI did not include zero.…”

“…To account for various sources of dependency between effect sizes within and across studies, we created a multilevel mixed effects meta‐analytic model using the nlme package in R (Van den Noortgate et al, 2013; Pinheiro et al, 2017). Effect sizes were treated as fixed effects, whereas random effects were specified in the nesting structure site/study/site‐year/controls to account for dependencies between multiple effect sizes sharing the same control and multiple effect sizes from the same site‐year, study, and experimental site (Thapa et al, 2018).…”

Can N2O emissions offset the benefits from soil organic carbon storage?

“…Increased C input but in some cases (e.g. manure) rather a transfer from one terrestrial location to another than a transfer of C from atmosphere to soil (Diacono & Montemurro, 2011) Enhanced denitrification rate (via anaerobiosis and the supply of electron donors), and soil N availability (Charles et al, 2017) Use of cover crops Reduced C loss/increased C input (Poeplau & Don, 2015) Decreased denitrification because of N uptake by plants; may be compensated for by N inputs from biological nitrogen fixation Thapa et al, 2018) Biochar Increased C input (Lehmann et al, 2006) Decreased nitrification due to adsorption of mineral N with biochar (Borchard et al, 2019) Agroforestry Increased C input, reduced C loss, increased aggregate stability (Feliciano et al, 2018) Decreased denitrification (lower soil moisture, increased soil porosity, increased nitrogen uptake), except for N 2 -fixing trees (increasing soil available N; Kim et al, 2016) 3 | HOW SO C S TORING PR AC TICE S A FFEC T N 2 O EMISS I ON S…”

“…In this scenario, the cumulative climate change mitigation effect of SOC sequestration was, on average, totally offset after 50 years since the adoption of cover crops, due to enhanced N 2 O emissions. While cover crops may induce higher N 2 O emissions, in particular if leguminous crops are extensively used, they can also reduce nitrate leaching, by about 56% on average (Thapa et al., 2018). This is beneficial for water quality and would be expected to lead to decreased indirect N 2 O emission through denitrification of nitrate entering surface water.…”