In tropical regions, climate conditions favor fast decomposition of soil organic matter (SOM), releasing into the soil organic composts in solid, liquid, and gaseous forms with variable compositions. Dissolved organic matter (DOM), a complex mixture of thousands of organic compounds, is only a small fraction of the decomposition products; however, it is highly mobile and reactive to the soil. Therefore, DOM play a key role in soil aggregation (formation of organometallic complexes), energy source for microorganisms, as well as C storage, cycling, and provision of plant-available nutrients. DOM multifunctionality to sustain soil functions and important ecosystem services have raised global scientific interest in studies on DOM fractions. However, previous studies were conducted predominantly under temperate soil conditions in natural ecosystems. Therefore, there is paucity of information on tropical soil conditions under agricultural systems, where DOM turnover is intensified by management practices. This review synthesized information in the literature to identify and discuss the main sources, transformations, and future of DOM in soils. We also discussed the importance of this fraction in C cycling and other soil properties and processes, emphasizing agricultural systems in tropical soils. Gaps and opportunities were identified to guide future studies on DOM in tropical soils.

Global demand for bioenergy increases interest in biomass‐derived fuels, as ethanol from sugarcane straw. However, straw is the main carbon source to soil and its removal reduces C input, affecting active fractions (dissolved organic carbon, DOC) and C storage. To quantify the effects of straw removal on DOC and C stocks, we built lysimeter system using soil (Rhodic Kandiudox) from sugarcane field. We evaluated four soil depths (1, 20, 50, and 100‐cm) and four straw removal rates: no‐removal‐NR, medium‐MR, high‐HR, and total‐TR, leaving 12, 6, 3, and 0 Mg ha^‐1 on the soil surface, respectively. After rainfall, drainage water was collected and analyzed for DOC content. Soil C stocks were determined after the 17‐month. Total DOC released at 1‐cm depth amounted to 606, 500, 441, and 157 kg ha^‐1 in NR, MR, HR, and TR, respectively. Net‐DOC suggests straw as the main source of DOC. Most of DOC in NR (50%) was retained within the 1‐20 cm layer, resulting in higher C stock (10 Mg ha^‐1) in the topsoil. In HR and MR, DOC retention was higher within 20‐50 cm, suggesting differences in DOC composition. DOC in TR was 40% higher at 20‐cm than at 1‐cm, indicating C losses from topsoil. Low concentrations of DOC were found at 100‐cm depth, but representing 30% in TR. Straw removal for bioenergy production is sustainable, but we should leave at least 3 Mg ha^‐1 of straw to ensure DOC production and soil C storage, taking account the DOC contribution to key soil functions.

Soil organic matter decomposition affects the local and global C cycles. Decomposition is mainly affected by soil type and climatic conditions, for a given quality of organic material. This study tested the effect of land use and management, litter type, and climate on the early stage decomposition rate in long-term field experiments (LTEs) in Austria and Sweden. Standardized litter (Rooibos and Green tea) were used according to the Tea Bag Index (TBI) protocol (Keuskamp et al. 2013) for comparison of litter decomposition rate (k) and stabilization factor (S) in 11 sites in Austria (2015 and 2016) and 9 sites in Sweden (2016). The tea bags were buried at 8 cm depth and collected after ~90 days. Austrian LTEs focused on mineral nitrogen fertilization, mineral potassium fertilization, organic fertilization, tillage systems, and crop residues management. The LTEs evaluated in Sweden focused mainly on annual and perennial crops, mineral fertilization, and tillage systems. The impact of environmental parameters (air temperature and precipitation) was modeled to normalize the variance due to climatic effects at each site. The preliminary results show that in Austria TBI decomposition differed more between sites than between treatments at the same LTE. Minimum tillage treatment had significantly higher decomposition rates compared to reduced and conventional tillage. In Sweden, decomposition rate differed more between treatments than between sites. Fertilized plots showed higher stabilization than unfertilized, and maximum N fertilization had the highest k, while unfertilized had the lowest k. The effect of different tillage systems on k and S were variable across sites and treatments, although ploughing tended to result in the lowest k. The northernmost site resulted in the highest k value. Results indicated higher stabilization in perennial forage crops compared to annual crops. We also considered time-series decomposition for some sites with measurements at different time points by the TBI approach (retrieving tea bags after 15, 30, 60, and 90 days), and the use of Random Forest regressions to evaluate the importance of pedo-climatic variables on early stage decomposition.