Cover crops do not increase soil organic carbon stocks as much as has been claimed: What is the way forward?

Abstract

When compared to virgin land (forest and grassland), croplands store significantly lower amounts of organic carbon (OC), mainly as a result of soil tillage, and decreased plant inputs to the soil over the whole year. Doubts have been expressed over how much reduced and zero tillage agriculture can increase OC in soils when the whole soil profile is considered. Consequently, cover‐crops that are grown in‐between crops instead of leaving soils bare appear as the “last man standing” in our quest to enhance cropland OC stocks. Despite the claim by numerous meta‐analyses of a mean carbon sequestration rate by cover crops to be as high as 0.32 ± 0.08 ton C ha−1 year−1, the present analysis showed that all of the 37 existing field studies worldwide only sampled to a depth of 30 cm or less and did not compare treatments on the basis of equivalent soil mass. Thirteen studies presented information on OC content only and not on OC stocks, had inappropriate controls (n = 14), had durations of 3 years or lower (n = 5), considered only one to two data points per treatment (n = 4), or used cover crops as cash crops (i.e., grown longer that in‐between two crops) instead of catch crops (n = 2), which in all cases constitutes shortcomings. Of the remaining six trials, four showed non‐significant trends, one study displayed a negative impact of cover crops, and one study displayed a positive impact, resulting in a mean OC storage of 0.03 ton ha−1 year−1. Models and policies should urgently adapt to such new figure. Finally, more is to be done not only to improve the design of cover‐crop studies for reaching sound conclusions but also to understand the underlying reasons of the low efficiency of cover crops for improved carbon sequestration into soils, with possible strategies being suggested.

Cover crop studies: The need for more reliable data

“…Our aim is not to discourage the practice of using CC in arable agriculture, which, as indicated in Chaplot and Smith (2023b), can have multiple benefits. Rather, it is to demonstrate potential pitfalls in meta-analyses, if they include studies that do not meet quality criteria.…”

“…Poeplau et al. (2023) have raised concerns regarding our recent contribution to Global Change Biology (Chaplot & Smith, 2023b) and conclude that cover crops (CC) do increase soil organic carbon stocks (SOCS).…”

“…Consequently, the claim that CC increases SOCS worldwide is based on very limited evidence. After excluding a number of studies on various quality criteria (such as the absence of bulk density data or control treatment at 29 sites; Chaplot & Smith, 2023b), there are too few studies to conclude a statistically positive impact of CC on SOCS.…”

Correction to “Cover crops do not increase soil organic carbon stocks as much as has been claimed: What is the way forward?”

“…In the paper by Chaplot and Smith (2023), we incorrectly reported that the study by Thomsen and Christensen (2004) reported on soil bulk density (BD). Our central conclusion, that current studies do not provide sufficient reliable data to conclude an impact of cover crops on soil organic carbon stocks, and our suggestions for future experiments remain unchanged.…”

Cover crops do increase soil organic carbon stocks—A critical comment on Chaplot and Smith (2023)

“…Beillouin et al (2023) performed a second-order global meta-analysis and identified five cover crop meta-analyses, showing a significant positive mean effect of cover crops on SOC (+11.6%). Obviously, international standards to carry out systematic reviews and meta-analyses (Gurevitch et al, 2018) were not followed by Chaplot and Smith (2023), which is a major shortcoming of this paper.…”