Search

• Irrigation and long-term cultivation (26 years) did not affect SOC stock. • SOC dynamics in the subsoil is particularly vulnerable to irrigated agriculture. • Land use favors large amounts of aliphatics products along the soil profile. • Management changes microbiological parameters, especially bacterial communities. • Changes in the bacterial community suggests alterations in groups of subsoil C. Changes in the global water cycle affect irrigation requirements and soil processes as well as the structure of soil microbial communities and function and the carbon (C) balance. The microbial structure and C sequestration under land use changing scenarios are rarely studied in the semi-arid region of Brazil. Here, we assessed the effects of land-use changes (irrigation and cultivation) on C storage in the soil and the structure of total bacteria (16S rRNA gene profile) and fungus (18S rRNA gene profile) at soil depth. We evaluated two soil profiles in the semi-arid region in northeastern Brazil. Profile P1 in an area with long-term irrigation (1,500 mm of water per year) and cultivation (small-scale farming systems, mainly with subsistence crops, which is a frequent land use system in the region of irrigated soil) for about 26 years. Profile P2 under natural conditions with 400 mm of water per year of rainfall (rain-fed hyperxerophilic Caatinga). Land-use changes showed a small influence on C storage; however, the changes strongly affected the soil organic matter (SOM) quality as well as the bacterial and fungal community structure, indicated by higher contents of labile-C and microbial biomass along with lower C-CO 2 emission. The relatively large amounts of aliphatic groups in the soil surface and subsurface in the cropped system (P1) suggested enrichment of methylene structures and probable increase of microbial-derived compounds. Methoxylic and aromatic groups tend to increase at depth (Bt horizon) indicating that the subsoil C responds to soil management changes. The structures of the bacterial community are distinct at depth in P1 and more similar at depth in P2. On the other hand, the structures of the fungal communities are more similar at depth in P1 than in P2. Furthermore, the irrigated agriculture also presented a carbon management index greater than 140% in most horizons, indicating that the soil management system did not cause soil degradation. [ABSTRACT FROM AUTHOR]

• Successful prediction of weathering indices with MIR spectroscopy. • Land use explains minor part of variations in soil thickness and weathering degree. • Soil thickness and weathering degree mainly determined by slope gradient. • Locally high erosion does not necessarily result in changes at landscape-scale. Land use and land cover changes (LUCC) can drastically alter various components of the critical zone, including soil thickness and soil chemical weathering processes. Often these studies, however, tend to focus on extreme cases, not representing what actually happens on average at larger, regional scales. Here, we evaluate the impact of LUCC on soil thickness and soil weathering degree at the regional scale, where we use soil spectroscopy to derive weathering indices. In a subtropical region in Southern Brazil, we collected calibration/validation soil samples (n = 49) from 4 different locations for which we measured the mid-infrared (MIR) spectral reflectance and 3 soil chemical weathering indices: chemical index of alteration (CIA), the total reserve in bases (TRB), and the iron ratio (Fe d /Fe t). We used partial least square regressions on this calibration/validation dataset to relate the MIR spectra of the soil samples to these weathering indices, resulting in good calibration relationships with R2 values of 0.97, 0.91 and 0.84 for CIA, TRB and Fe d /Fe t , respectively. Applying these relations to MIR spectra of regionally collected soil samples allowed us to calculate soil weathering degrees for a large number of soil samples (n = 229), without requiring costly and time-consuming chemical analyses. We collected these soil samples at 100 mid-slope positions: 50 under forest and 50 under agricultural land use. Land use explained only a minor part of the variation in soil thickness and weathering degree. Thus, while local water and tillage erosion rates might be considerable after deforestation, this has not led to significant reductions in average soil thickness and has not affected soil weathering degree. Slope gradient is the main factor influencing the spatial variability in soil thickness and weathering degree on mid-slope sections in our study area. Human activities over the last century did not fundamentally alter these patterns. [ABSTRACT FROM AUTHOR]