Major limitations to achieving “4 per 1000” increases in soil organic carbon stock in temperate regions: Evidence from long‐term experiments at Rothamsted Research, United Kingdom

Abstract

We evaluated the “4 per 1000” initiative for increasing soil organic carbon (SOC) by analysing rates of SOC increase in treatments in 16 long‐term experiments in southeast United Kingdom. The initiative sets a goal for SOC stock to increase by 4‰ per year in the 0–40 cm soil depth, continued over 20 years. Our experiments, on three soil types, provided 114 treatment comparisons over 7–157 years. Treatments included organic additions (incorporated by inversion ploughing), N fertilizers, introducing pasture leys into continuous arable systems, and converting arable land to woodland. In 65% of cases, SOC increases occurred at >7‰ per year in the 0–23 cm depth, approximately equivalent to 4‰ per year in the 0–40 cm depth. In the two longest running experiments (>150 years), annual farmyard manure (FYM) applications at 35 t fresh material per hectare (equivalent to approx. 3.2 t organic C/ha/year) gave SOC increases of 18‰ and 43‰ per year in the 23 cm depth during the first 20 years. Increases exceeding 7‰ per year continued for 40–60 years. In other experiments, with FYM applied at lower rates or not every year, there were increases of 3‰–8‰ per year over several decades. Other treatments gave increases between zero and 19‰ per year over various periods. We conclude that there are severe limitations to achieving the “4 per 1000” goal in practical agriculture over large areas. The reasons include (1) farmers not having the necessary resources (e.g. insufficient manure); (2) some, though not all, practices favouring SOC already widely adopted; (3) practices uneconomic for farmers—potentially overcome by changes in regulations or subsidies; (4) practices undesirable for global food security. We suggest it is more realistic to promote practices for increasing SOC based on improving soil quality and functioning as small increases can have disproportionately large beneficial impacts, though not necessarily translating into increased crop yield.

Deep soil inventories reveal that impacts of cover crops and compost on soil carbon sequestration differ in surface and subsurface soils

“…Our study represents one of few long-term efforts to track soil carbon changes throughout both the surface and subsoil layers in (Périé & Ouimet, 2008). Poulton et al (2018) also observed declines in bulk density in longterm cropping system experiments.…”

“…Bulk density likely declined in the surface layers of most systems due to the cumulative addition of organic matter, which built up as bulky or particulate organic matter; others have observed an inverse relationship between soil organic matter and bulk density (Périé & Ouimet, ). Poulton et al () also observed declines in bulk density in long‐term cropping system experiments.…”

“…A global meta-analysis of 30 studies found that cover crops increase soil C stocks by 0.32 Mg C ha −1 year −1 but was limited to the top 30 cm (Poeplau & Don, 2015). In contrast, Poulton et al (2018) observed losses of 0.55 Mg C ha −1 year −1 in temperate annual cropping systems with winter cover crops (WCC). Studies more commonly report increases rather than decreases in soil C with WCC.…”

“…Crop management practices that increase long-term C include cultivation of perennial crops and/or cultivation of pastures (Armstrong, Millar, Halpin, Reid, & Standley, 2003;Follett & Reed, 2010;Lal, 2004;Sanford et al, 2012) and applications of organic amendments (e.g., manure, compost) (Brar, Singh, Dheri, & Kumar, 2013;Poulton, Johnston, Macdonald, White, & Powlson, 2018;Zhang et al, 2012). Net C losses, however, can result from excessive tillage, overgrazing and fallowing (Hernanz, Sanchez-Giron, & Navarrete, 2009;Maia, Ogle, Cerri, & Cerri, 2009).…”

Converting temperate long‐term arable land into semi‐natural grassland: decadal‐scale changes in topsoil C, N, 13C and 15N contents

“…The international 4‰ initiative, launched at the COP‐21 conference in Paris (Minasny et al ., ) as a means to combat increasing atmospheric concentrations of CO 2 , aims to increase C content in the top 40 cm of the soil by 0.4% annually by improved land management and land‐use change. Although the quantitative realism of this initiative has been questioned (Poulton et al ., ), increased soil C storage will counteract, at least for a period, the increase in atmospheric CO 2 concentration. One option to store more C in soil is to revert degraded and arable lands with small C content and limited productivity to permanently vegetated soils such as semi‐natural grasslands (Powlson et al ., ).…”