Free and protected soil organic carbon dynamics respond differently to abandonment of mountain grassland

[摘要] Land-use change (LUC) and management are among the major driving forces ofsoil carbon (C) storage. Abandonment of mountain grassland promotesaccumulation of aboveground biomass and litter, but related responses of soilorganic matter (SOM) dynamics are uncertain. To determine SOM-C turnover wesampled 0–10 cm of soils in the European Alps along two land-use gradients (hay meadows, grazedpastures and abandoned grasslands) of different managementintensity. A first land-use gradient was located at Stubai Valley (MAT: 3 °C, MAP: 1097 mm) in Austria anda second at Matsch Valley (MAT: 6.6 °C, MAP: 527 mm) in Italy. We estimated Cinput and decomposition rates ofwater-floatable and free particulateorganic matter (wPOM, fPOM <1.6 g cm⁻³) andaggregate-occluded particulate and mineral-associated organic matter (oPOM<1.6 g cm⁻³, mOM >1.6 g cm⁻³) using bomb radiocarbon.

In mountain grasslands average C turnover increased from roots (3 yr) < wPOM (5 yr) < fPOM (80 yr) < oPOM (108 yr) < mOM(192 yr). Among SOM fractions the turnover of fPOM-C varied most in relation to management. Along both land-use gradients C input pathways shifted from root-derivedtowards litter-derived C. The C input rates of both wPOM-C and fPOM-C were affected by land management at both sites. In contrast,oPOM-C and mOM-C dynamicsremained relatively stable in response to grassland abandonment. Carbon accumulation rates of free POM decreased strongly with time since LUC (10, 25 and 36 yr).For wPOM-C, for example, it decreased from 7.4> 2.2> 0.8 g C m⁻² yr⁻¹. At both sites, most C was sequestered in the first years afterLUC and free POMreached new steady state within 20–40 yr.

We conclude that w-and fPOM-C vs. oPOM-C dynamics respond differently tograssland management change and thus POM does not represent a homogeneous SOMfraction. Sequestered C is stored in the labile POM and not stabilized in the long-term.Thus, it is unlikely thatabandonment, the dominant form of LUC in the European Alps, provides asubstantial net soil C sink.

[学科分类] 地球化学与岩石