Interactions between biogeochemical and management factors explain soil organic carbon in Pyrenean grasslands

Abstract

Abstract. Grasslands are one of the major sinks of terrestrial soil organic carbon (SOC). Understanding how environmental and management factors drive SOC is challenging because they are scale-dependent, with large scale drivers affecting SOC both directly and through drivers working at detailed spatial scales. Here we addressed how regional, landscape and grazing management, soil properties and nutrients and herbage quality factors affect SOC in mountain grasslands in the Pyrenees. Taking advantage of the high variety of environmental heterogeneity in the Pyrenees, we fit a set of models with explicative purposes using data that comprise a wide range of environmental and management conditions. We found that temperature seasonality (MMT) was the most important geophysical driver of SOC in our study. MMT was positively related to SOC but only under certain local conditions: exposed hillsides, steep slopes and relatively highly grazed areas. High MMT conditions probably are more favourable for plant biomass production, but landscape and grazing management factors buffer the accumulation of this biomass into SOC. Concerning biochemical SOC predictors, we obtained some surprising, interactive effects between grazer type, soil nutrients and herbage quality. Soil N was a crucial factor modulating effects of livestock species and neutral detergent fibre content of plant biomass and herbage recalcitrance effects varied depending on grazer species. These results highlight the gaps in the knowledge about SOC drivers in grassland under different environmental and management conditions, and they may serve to generate testable hypothesis in latter studies directed to climate change mitigation policies.

Plant Functional Diversity, Climate and Grazer Type Regulate Soil Activity in Natural Grasslands

“…We used CANOCO 5 for all the analyses [54]. The explanatory sets initially included all the variables recorded in the study: (a) environmental (climatic, management, and soil descriptors) variables: mean annual air temperature (MAT), mean annual precipitation (MAP), mean annual minimum air temperature (MTmin), mean annual maximum air temperature (MTmax), mean summer air temperature (MST), mean summer precipitation (MSP), the Temperature Seasonality Index of Sebastià (TSIS = MST − MAT; see Table 1), as in Rodríguez et al 2020 [55], grazer type (represented through two dummy variables, sheep grazing and cattle grazing), and pH and moisture determined for each sampled soil; (b) plant functional diversity variables: PFT evenness, biomass proportion of grasses, legume, and non-legume forbs (thereupon, legumes and forbs), and the pairwise interactions between the three plant PFTs. We did not have enough degrees of freedom to reliably include interactions between PFT diversity components and other environmental variables in our redundancy analysis.…”

Positive Effects of Legumes on Soil Organic Carbon Storage Disappear at High Legume Proportion Across a Wide Range of Environmental Conditions in Grasslands in the Pyrenees

“…The organic C fraction was determined by subtracting inorganic C in the carbonates from the total C. Soil organic carbon stocks (SOC) in the upper 20 cm soil layer were then estimated taking into account the organic C concentration in the sample and its bulk density, and subtracting the coarse particle (> 2 mm) content, following García-Pausas et al (2007). See Rodriguez et al 2020 for further details about SOC sampling and determination.…”