A critical review of the impacts of cover crops on nitrogen leaching, net greenhouse gas balance and crop productivity

Abstract

Cover crops play an increasingly important role in improving soil quality, reducing agricultural inputs and improving environmental sustainability. The main objectives of this critical global review and systematic analysis were to assess cover crop practices in the context of their impacts on nitrogen leaching, net greenhouse gas balances (NGHGB) and crop productivity. Only studies that investigated the impacts of cover crops and measured one or a combination of nitrogen leaching, soil organic carbon (SOC), nitrous oxide (N₂ O), grain yield and nitrogen in grain of primary crop, and had a control treatment were included in the analysis. Long-term studies were uncommon, with most data coming from studies lasting 2-3 years. The literature search resulted in 106 studies carried out at 372 sites and covering different countries, climatic zones and management. Our analysis demonstrates that cover crops significantly (p < 0.001) decreased N leaching and significantly (p < 0.001) increased SOC sequestration without having significant (p > 0.05) effects on direct N₂ O emissions. Cover crops could mitigate the NGHGB by 2.06 ± 2.10 Mg CO₂ -eq ha^-1 year^-1 . One of the potential disadvantages of cover crops identified was the reduction in grain yield of the primary crop by ≈4%, compared to the control treatment. This drawback could be avoided by selecting mixed cover crops with a range of legumes and non-legumes, which increased the yield by ≈13%. These advantages of cover crops justify their widespread adoption. However, management practices in relation to cover crops will need to be adapted to specific soil, management and regional climatic conditions.

Keywords: C sequestration; N content; N in grain; N leaching; catch crop; cover crop; green manure; net greenhouse gas balance; nitrate; nitrous oxide emissions; soil organic carbon; yield.

Figure 1

Map showing the net primary productivity (NPP) and locations of experimental sites considered in this paper. NPP calculated using the Miami method (Grieser et al., 2006; Leith, 1972)

Figure 2

Comparisons between N leaching (a), grain yield (b), N in grain (c) and SOC (d) from control and cover crops (CC) treatments. Types of cover crops (legume [blue], non‐legume [green] or mixed [red]) and their 95% confidence intervals

Figure 3

Relationships between change in N leaching (%) and legume, non‐legume and mixed cover crops. On the y‐axis are parameter estimates of N leaching based on a linear mixed effects model with added N fertilizer as a covariate (n = 66; p > 0.3; vertical bars denote 95% confidence intervals)

Figure 4

Contour plot (n = 38) showing relationships between added N fertilizer application rate, bulk density (BD) and change in N leaching (%) at 0–100 cm soil depth. These two variables explain 11.6% of N leaching overall variation (p < 0.05). N leaching significantly depended on BD (t = 2.62; p < 0.05). One outlier was removed (BD = 2.5)

Figure 5

Contour plot (n = 41) showing relationships between added N fertilizer application rate, bulk density (BD) and change in soil organic carbon (SOC) (%). Added N fertilizer and BD explain 57% of SOC overall variation (p < 0.001). The SOC depended significantly on added N (t = −3.2; p < 0.01) and BD (t = 7.1; p < 0.001)

Figure 6

Contour plot (n = 35) showing relationships between added N fertilizer application rate, pH and change in soil organic carbon (SOC) (%). Added N fertilizer and pH explain 31% of SOC overall variation (p < 0.001). SOC depended significantly on pH (t = 3.94; p < 0.001)

Figure 7

Relationships between soil organic carbon (SOC) and mean annual air temperature (MAAT) (a) and mean annual precipitation (MAP) (b) under control and cover crops. MAAT was not significantly correlated with SOC (p > 0.05). MAP was positively correlated with SOC for both the control (t = 5, p < 0.001; r ² = 0.39, p < 0.001, n = 43), and cover crop (t = 5, p < 0.001; r ² = 0.39, p < 0.001, n = 43)