Soil macroaggregate-occluded mineral-associated organic carbon drives the response of soil organic carbon to land use change

Understanding land use effects on carbon sequestration in various soil fractions is vital to mitigating climate change and restoring soil functions. The objective of this study was to explore the effects of land use on soil organic carbon (SOC) fractions in different soil types. For this purpose, we studied the effects of long-term (>20 years) land use including dryland pasture (DP), irrigated pasture (IP) and irrigated cropland (IC) on SOC in water-stable aggregates, particle-size fractions, and their coupling relations at the surface soils (0–7.5 cm) in the Canterbury Plains, New Zealand. For each land use, three typical soil types with contrasting drainage levels (i.e. well drained Lismore soil, LIS; imperfectly drained Templeton soil, TEM; and poorly drained Waterton/Temuka soil, WAT) were selected. Macroaggregate-occluded mineral-associated organic carbon (M-MAOC) contributed to the majority of the total SOC difference and drove the response of SOC to land use change. On average, M-MAOC followed an order of IP > DP > IC. The effects of land use change from DP to IP and IC on M-MAOC varied, and these variations were dependent on soil type. The relative gain in M-MAOC with change in land use from DP to IP was the greatest in the well drained LIS soil, while both the relative and absolute loss in M-MAOC following the land use change to IC was the greatest in the poorly drained WAT soil. The interactive effects of managements (e.g. irrigation and cultivation) and soil type (e.g. soil water condition) on aggregate size distribution and macroaggregate-associated C concentration were important in explaining the responses of M-MAOC to land use change. This study advances the mechanistic understanding of total SOC dynamics in response to land use (changes) in different soil types. It also highlights the potential of M-MAOC to serve as a diagnostic fraction to reflect changes in total SOC, which may have application to global warming mitigation.