Soil organic matter dynamics in long-term temperate agroecosystems: rotation and nutrient addition effects

Abstract

Soil organic matter (SOM) is a major driver of key agroecosystem functions. Our objective was to examine the dynamics of organic matter in whole soil, particulate (POM; >53 μm size), and mineral-associated (MAOM) fractions under varying crop rotations and nutrient managements at two long-term experimental sites (Breton and Lethbridge). Soil samples were collected from simple (2 yr) and complex (5 or 6 yr) crop rotations at the 5-10 cm depth. We found associations between SOM pools versus microbial community and soil aggregation. Compared to cropped soils, an adjacent forest exhibited a significantly higher soil total organic carbon (TOC) and a shift in SOM fractions with substantially higher POM. However, the forest soil had the lowest microbial biomass C among all the assessed land use systems (P < 0.05), suggesting that other factors than the amount of labile SOM (i.e., POM-C) were controlling the microbial community. When contrasted to simple 2 yr rotations, the complex rotations including perennials and legumes significantly raised TOC and soil total nitrogen as well as the stable SOM fraction (i.e., MAOM-C and -N) consistently for both Breton and Lethbridge sites. Our findings highlight that varying land managements have profound feedbacks on soil quality as mediated by alterations in long-term SOM dynamics.Résumé : La matière organique (MO) joue un rôle important dans le fonctionnement des écosystèmes agricoles. Les auteurs voulaient approfondir la dynamique de la MO dans le sol entier, dans les particules de matière organique (PMO; granulométrie > 53 μm) et dans les minéraux associés à la matière organique (MAMO) en fonction du régime d'assolement et de gestion des éléments nutritifs à deux sites expérimentaux à long terme (Breton et Lethbridge). Pour cela, ils ont échantillonné le sol des assolements simples (2 ans) et complexes (5 ou 6 ans) à une profondeur de 5 à 10 cm. Le réservoir de MO présente des liens avec la microflore tellurique et les agrégats. Comparativement aux sols cultivés, le sol d'une forêt contigüe s'est avéré sensiblement plus riche en carbone organique total (COT), et la MO comprenait sensiblement plus de particules. Cependant, le sol forestier présentait la plus faible biomasse microbienne parmi les sols examinés (P < 0,05), signe que d'autres facteurs que la concentration de MO labile (à savoir, le C-PMO) régissent la microflore. Comparativement aux assolements simples de deux ans, les assolements complexes incluant des vivaces et des légumineuses accroissent sensiblement le COT et l'azote total du sol, de même que la fraction de MO stable (à savoir, C-MAMO et N-MAMO), tant aux sites de Breton qu'à ceux de Lethbridge. Ces constatations soulignent que les différents régimes de gestion du sol ont des répercussions appréciables sur la qualité du sol, consécutivement à une modification de la dynamique à long terme de la MO. [Traduit par la Rédaction] Mots-clés : écosystème agricole, biomasse microbienne, matière organique du sol, agrégation, qualité du sol.

Dynamics of Soil Carbon Concentrations and Quality Induced by Agricultural Land Use in Central South Africa

“…Secondary grassland management provided a permanent cover to control erosion‐induced losses of SOC and SIC, which were suspected to have occurred in the sampled agro‐ecosystems (Lobe et al, 2001, 2002). Exclusion of intensive cultivation in the secondary grasslands reduced SOC decomposition rates and stabilized SIC dissolution‐precipitation processes (Wang et al, 2016; Kiani et al, 2017; Li et al, 2018). Minimum decomposition under secondary grassland management is a benefit in sandy soils with low C protective capacity (Preger et al, 2010; Lauer et al, 2011; Kösters et al, 2013).…”

“…Therefore a decline in POXC suggests continuous exposure of organic materials to the decomposer community in the prolonged cultivated soils or a constant supply of large quantities of labile inputs in the virgin soils. Possibly, constant applications of N containing fertilizers in the cultivated soils also accelerated decomposition rates of POXC compared to unfertilized virgin soils (Li et al, 2018).…”

“…Neither initial saturation levels of soil C fractions in the former cultivated soils nor climate and clay content affected POXC restoration (Table 5). Annual N applications likely improved phytomass production that subsequently replenished the lost POXC by cultivation and provided protection against erosion, especially in Harrismith where N application rates were at least higher than in Tweespruit and Kroonstad (Kiani et al, 2017; Li et al, 2018). On the other hand, higher N applications in Harrismith constrained microbial activity and therefore spared POXC from loss (Gregorich et al, 1994).…”

“…However, there are still conflicting reports on the effects of this management shift on soil C quantity and quality. Crop rotation that included perennial pastures and legumes were reported to have positive effects on overall soil quality in the temperate climates of Canada (Kiani et al, 2017; Li et al, 2018). On the other hand, soil C did not show significant responses following 150‐yr grassland restoration sequence in the Loess Plateau of China (Wang et al, 2016).…”

“…Thus, examining changes in specific fractions reveals the nature of accumulating C, which is crucial and of course a prerequisite for effective management of soil C in agricultural based production ecosystems (Janzen, 2005). Therefore, studies of this kind are still in great need because soil C fractions have different response timeframes to land use change, and are not fully understood due to variable site‐specific conditions (Gregorich et al, 1994; Janzen, 2005, 2006; Li et al, 2018). More importantly, such studies are necessary for assessing sustainability of management strategies and decisively developing soil protection guidelines and policies (Gregorich et al, 1994; Kotzé et al, 2016; Vázquez et al, 2016).…”