Soil organic carbon in irrigated agricultural systems: A meta‐analysis

Abstract

Over the last 200 years, conversion of non‐cultivated land for agriculture has substantially reduced global soil organic carbon (SOC) stocks in upper soil layers. Nevertheless, practices such as no‐ or reduced tillage, application of organic soil amendments, and maintenance of continuous cover can increase SOC in agricultural fields. While these management practices have been well studied, the effects on SOC of cropping systems that incorporate irrigation are poorly understood. Given the large, and expanding, agricultural landbase under irrigation across the globe, this is a critical knowledge gap for climate change mitigation. We undertook a systematic literature review and subsequent meta‐analysis of data from studies that examined changes in SOC on irrigated agricultural sites through time. We investigated changes in SOC by climate (aridity), soil texture, and irrigation method with the following objectives: (i) to examine the impact of irrigated agriculture on SOC storage; and (ii) to identify the conditions under which irrigated agriculture is most likely to enhance SOC. Overall, irrigated agriculture increased SOC stocks by 5.9%, with little effect of study length (2–47 years). However, changes in SOC varied by climate and soil depth, with the greatest increase in SOC observed on irrigated semi‐arid sites at the 0–10 cm depth (14.8%). Additionally, SOC increased in irrigated fine‐ and medium‐textured soils but not coarse‐textured soils. Furthermore, while there was no overall change to SOC in flood/furrow irrigated sites, SOC tended to increase in sprinkler irrigated sites, and decrease in drip irrigated sites, especially at depths below 10 cm. This work sheds light on the nuances of SOC change across irrigated agricultural systems, highlights the importance of studying SOC storage in deeper soils, and will help guide future research on the impacts of irrigated agriculture on SOC.

Effects of land clearing for agriculture on soil organic carbon stocks in drylands: A meta‐analysis

“…Otherwise, higher mean SOC stocks before clearing in forest than in perennial crops which is also likely a major contributor to the SOC reduction (Negash et al, 2022). Surprisingly, we found that irrigation did not significantly affect the response of SOC stocks to land clearing, which is inconsistent with studies that have found irrigation in water‐limited systems increases plant productivity and alters the SOC sequestration processes, potentially resulting in a 5.9% increase in SOC stocks (Emde et al, 2021; Wu et al, 2008). We suspect that the effects of irrigation on SOC changes were masked by the lack of consistent irrigation consumption data from the raster layers since only 61 of 242 sampling sites derived from the literature reported irrigation water consumption.…”

“…Otherwise, higher mean SOC stocks before clearing in forest than in perennial crops which is also likely a major contributor to the SOC reduction (Negash et al, 2022). Surprisingly, we found that irrigation did not significantly affect the response of SOC stocks to land clearing, which is inconsistent with studies that have found irrigation in water-limited systems increases plant productivity and alters the SOC sequestration processes, potentially resulting in a 5.9% increase in SOC stocks (Emde et al, 2021;.…”

“…As mentioned above, the sizes of CNEC effects on SOC stocks depend on the coupled contributions of initial natural ecosystem types and cropland management practices (Alegre & Cassel, 1986; Emde et al, 2021; Smith, 2007; VandenBygaart et al, 2003). However, among the selected studies, only 33.88% of sampling sites reported fertilizer consumption, 25.21% of them reported irrigation consumption, and 90.08% of sites reported cultivation duration.…”

“…As mentioned above, the sizes of CNEC effects on SOC stocks depend on the coupled contributions of initial natural ecosystem types and cropland management practices (Alegre & Cassel, 1986;Emde et al, 2021;Smith, 2007;VandenBygaart et al, 2003).…”

A well‐established fact: Rapid mineralization of organic inputs is an important factor for soil carbon sequestration

“…There are also examples showing that combining cover crops and no‐tillage, without exogenous organic inputs, can result in even much larger SOC sequestration rates, for example, in Southern Brazil (Veloso et al, 2018) or in France (Autret et al, 2016). Other practices or systems that can improve SOC levels include agroforestry systems (Cardinael et al, 2018; Corbeels et al, 2019; Mayer et al, 2022), the addition of available exogenous organic matter that would otherwise not be applied to soil (Bruni et al, 2022; Maillard & Angers, 2014), and finally, the removal of soil constraints that result in increased crop productivity and thus C inputs from the crop itself, including roots (Emde et al, 2021; Ladha et al, 2011). Obviously, the impact of these management practices on other greenhouse gas emissions (particularly N 2 O) has to be carefully accounted for.…”