No abstract

Soil capacity as a major carbon (C) sink is influenced by land use. Estimates of soil organic carbon (SOC) sequestration have mostly focused on topsoils [0–30 cm official Intergovernmental Panel on Climate Change (IPCC) soil depth]. We investigated SOC stocks and their quality as influenced by land-use changes. Soil samples were collected from five soil depths down to 100 cm of three adjacent fields each representing a different land use—forest, cassava, and rice paddy—in Northeast Thailand. Sequestration of SOC in topsoils under all land uses was higher, as indicated by SOC stocks (59.0–82.0 Mg ha−1) than subsoils (30–100 cm) (27.0–33.0 Mg ha−1). The soil profile (0–100 cm) of the forest had higher stocks of SOC and humic acid (115.0 and 6.8 Mg ha−1, respectively) than those of cultivated land uses [paddy (100.0 and 4.8 Mg ha−1, respectively) and cassava (87.0 and 2.3 Mg ha−1, respectively)], which accounted for an average 30% increase in SOC sequestration over those with only topsoil. Topsoils of the forest had higher humic acid content but narrower E4:E6 ratio [the ratio of absorbances at 465 nm (E4) and at 665 nm (E6)] of humic acids (2.8), indicating a higher degree of humification and stabilization than the cultivated soils (3.2–3.6). Subsoil C was higher quality, as indicated by the lower E4:E6 ratio of humic acids than topsoils in all land uses.

Against the background of current understanding of dissolved organic carbon (DOC) adsorption onto clay surfaces, it remains unclear if bulk DOC or its fractions contribute to microaggregate formation in the top layers of coarse-textured soils. We therefore investigated the effects of long-term inputs of biochemically contrasting organic residues on the chemical characteristics and vertical distribution of DOC in a coarse-textured Ultisol. During 2007-2008, DOC samples were extracted from soil profiles of a long-term residue quality field experiment initiated in 1995. In this field experiment, groundnut stover, dipterocarp and tamarind leaf litter, as well as rice straw of contrasting biochemical quality, were applied yearly at 10 Mg ha −1 . Groundnut, dipterocarp and tamarind produced large amounts (7.1-11.8 g C m −2 ) of high-molecular-weight (HMW; > 10 kDa) DOC, which was found in high concentrations (30-50 mg C kg −1 ) in the topsoil (0-15 cm). Rice straw, however, produced large amounts (3.5 g C m −2 ) of low-molecular-weight (LMW; < 1 kDa) DOC during the initial stage of decomposition. Although the HMW DOC was retained in the topsoil (0-15 cm), the LMW DOC was rapidly translocated to lower soil depths (60-80 cm). This translocation was facilitated by the low adsorption potential of the rice straw-derived LMW DOC on colloidal surfaces of the topsoil. There was a significant positive correlation of C in the HMW DOC with that in fine particles, indicating their contribution to microaggregate formation and thus C accumulation. It was concluded that biochemical quality of residues as a determinant of concentration and chemistry of DOC and its vertical dynamics along the soil profile must be considered for SOC accumulation in coarse-textured soils. Furthermore, we found reasonable indications that HMW DOC contributes to microaggregate formation in topsoils. Highlights• Residue quality determined vertical dynamics of DOC in coarse-textured Ultisols. • Lignin-and polyphenol-rich residues produced HMW DOC in topsoil.• LMW DOC derived from cellulose-rich residues was translocated to the subsoil.

The objectives of this study were to investigate effects of land use on accumulation of soil organic matter (SOM) in the soil profile (0–100 cm) and to determine pattern of SOM stock distribution in soil profiles. Soil samples were collected from five soil depths at 20 cm intervals from 0 to 100 cm under four adjacent land uses including forest, cassava, sugarcane, and paddy lands located in six districts of Maha Sarakham province in the Northeast of Thailand. When considering SOM stock among different land uses in all locations, forest soils had significantly higher total SOM stocks in 0–100 cm (193 Mg·C·ha−1) than those in cassava, sugarcane, and paddy soils in all locations. Leaf litter and remaining rice stover on soil surfaces resulted in a higher amount of SOM stocks in topsoil (0–20 cm) than subsoil (20–100 cm) in some forest and paddy land uses. General pattern of SOM stock distribution in soil profiles was such that the SOM stock declined with soil depth. Although SOM stocks decreased with depth, the subsoil stock contributes to longer term storage of C than topsoils as they are more stabilized through adsorption onto clay fraction in finer textured subsoil than those of the topsoils. Agricultural practices, notably applications of organic materials, such as cattle manure, could increase subsoil SOM stock as found in some agricultural land uses (cassava and sugarcane) in some location in our study. Upland agricultural land uses, notably cassava, caused high rate of soil degradation. To restore soil fertility of these agricultural lands, appropriate agronomic practices including application of organic soil amendments, return of crop residues, and reduction of soil disturbance to increase and maintain SOM stock, should be practiced.

Abstract. In light of the large role that soil organic matter (SOM) plays in maintaining healthy and productive agricultural soils, it is crucial to understand the processes of SOM protection including the role of soil aggregate protection. Yet, few numerical process models include aggregate formation and even fewer represent the important connection between microbial growth and aggregate formation. Here, we propose a model of Soil Aggregation through Microbial Mediation (SAMM), which consist of measurable pools and 5 couples soil aggregate formation to microbial growth. The model was evaluated against data from a long term bare-fallow experiment in a tropical sandy soil, subject to plant litter additions of different compositions. The SAMM model effectively represented the microbial growth response after litter addition and the following formation and later disruption of aggregates. Model parameter correlation was low (all r < 0.5; r > 0.4 for only 4 of 22 parameters) showing that SAMM is well parameterized. Differences between treatments resulting from different litter compositions could be captured by SAMM for soil organic carbon (Nash-Sutcliffe modelling efficiency (EF) of 0.68), microbial nitrogen (EF of 0.24) and litter carbon (EF of 0.80). Aggregate-related fractions, i.e., carbon inside aggregates (EF of 0.60) and also carbon in the free silt and clay fraction (EF of 0.24) were simulated very well to satisfactory. Analysis of model parameters led to further noteworthy insights. For example, model results suggested that up to 50 % of carbon in the soil is stabilized through aggregate protection, even in a sandy soil, and that both microbial activity and physical aggregate formation coexist. When aggregate formation was deactivated, the model failed to stabilize soil organic carbon (EF dropped to -3.68) and microbial nitrogen was represented less well (EF of 0.13). By re-calibrating the model version with deactivated aggregates, it was possible to partly correct for removing the aggregate formation, i.e., by reducing the decomposition rate of mineral attached carbon by about 85 % (EF of 0.68, 0.75 and 0.18 for SOC, litter carbon and microbial nitrogen, respectively). Yet, the overall slightly better evaluation statistics (e.g., Akaike information critereon of 5351 vs 5554) show the potential importance of representing aggregate dynamics within SOM models. Our results indicate that current models without aggregate formation partly compensate the missing protection effect by lowering turnover rates of other pools and thus may still be suitable options where data on aggregate associated carbon is not available.

&lt;p&gt;It is crucial to understand what influences the dynamics of soil aggregates, because soil organic matter (SOM) stabilized inside aggregates is the fraction of SOM that is most susceptible to anthropogenic activity. Yet, there is a lack of numerical process models that include the dynamics of aggregate formation and breakdown and to date, no model represents the important connection between microbial carbon use efficiency (CUE) and aggregate formation. Here, we introduce a model of microbially mediated aggregate formation, which includes litter-stoichiometry and -quality dependent CUE and simulates soil aggregate formation facilitated by the microbial excretion of binding substances. The model is evaluated against measured data of microbial biomass, SOM content and intra-aggregate SOM from a long term bare-fallow experiment in a tropical sandy soil, which was subject to plant litter addition of different qualities. The benefit of simulating aggregates in a model of SOM dynamics is assessed by comparing it against a version that does not, both being separately calibrated to the same dataset. Our results show that the developed model can effectively represent the microbial growth response that follows litter addition and the formation as well as the delayed breakdown of soil aggregates, after the microbial growth peaked. As shown by a higher modelling efficiency and a lower Akaike information criterion, the model version that includes aggregate formation outperforms the one that does not in the simulation of total organic carbon, total N and for the decomposition of litter. Additionally, it can represent the temporal dynamics of C stored in the silt and clay fraction. Yet, while the model could capture the temporal dynamic of aggregates as a result of litter quality, the amount of C in aggregates in the control treatment without litter addition was underestimated. Our results suggest that aggregate formation is an important process that could be included into SOM models to improve the simulation of both aggregated and non-aggregate pools. However, the underestimation of aggregate C in the control could be a hint, that abiotic aggregate formation may be a relevant factor, especially in low input systems, and may also need to be included.&lt;/p&gt;