Land use impact on carbon mineralization in well aerated soils is mainly explained by variations of particulate organic matter rather than of soil structure
Authors: Schlüter Steffen , Roussety Tim , Rohe Lena , Guliyev Vusal , Blagodatskaya Evgenia , Reitz Thomas 
Published: April 2022

Abstract. Land use is known to exert a dominant impact on a range of
essential soil functions like water retention, carbon sequestration, organic
matter cycling and plant growth. At the same time, land use management is
known to have a strong influence on soil structure, e.g., through
bioturbation, tillage and compaction. However, it is often unclear whether
the differences in soil structure are the actual cause of the differences in soil
functions or if they only co-occur. This impact of land use (conventional and organic farming, intensive and
extensive meadow, extensive pasture) on the relationship between soil
structure and short-term carbon mineralization was investigated at the
Global Change Exploratory Facility, in Bad Lauchstädt, Germany. Intact
topsoil cores (upper 10 cm, n=75) were sampled from all land use types at
the early growing season. Soil structure and microbial activity were
measured using X-ray-computed tomography and respirometry, respectively. Differences in microstructural properties between land uses were small in
comparison to the variation within land uses. The most striking difference
between land uses was larger macropore diameters in grassland soils due to
the presence of large biopores that are periodically destroyed in croplands.
Grasslands had larger amounts of particulate organic matter (POM), including
root biomass, and also greater microbial activity than croplands, both in
terms of basal respiration and rate of carbon mineralization during growth.
Basal respiration among soil cores varied by more than 1 order of
magnitude (0.08–1.42 µg CO2-C h−1 g−1 soil) and was
best explained by POM mass (R2=0.53, p<0.001).
Predictive power was only slightly improved by considering all bulk, microstructure
and microbial properties jointly. The predictive power of image-derived
microstructural properties was low, because aeration did not limit carbon
mineralization and was sustained by pores smaller than the image resolution
limit (<30 µm). The frequently postulated dependency of
basal respiration on soil moisture was not evident even though some cores
were apparently water limited, as it was likely disguised by the
co-limitation of POM mass. This finding was interpreted in regards to the microbial
hotspots which form on decomposing plant residues and which are decoupled
from water limitation in bulk soil. The rate of glucose mineralization
during growth was explained well by substrate-induced respiration
(R2=0.84) prior to growth, which in turn correlated with total
microbial biomass, basal respiration and POM mass, and was not affected by pore
metrics. These findings stress that soil structure had little relevance in predicting
carbon mineralization in well-aerated soil, as mineralization appeared to by
predominantly driven by the decomposition of plant residues in intact soil.
Land use therefore affects carbon mineralization in well-aerated soil mainly
in the amount and quality of labile carbon.