How feedback loops between meso- and macrofauna and organic residues contrasting in chemical quality determine decomposition dynamics in soils.

The concept of feedback loops between changes in chemical quality of decomposing organic residues and changes in faunal communities was employed in studying how such feedback loops, representing distinct ecological successional stages, determine decomposition dynamics in soils. A 52-week litterbag decomposition study was superimposed onto an 18-year long term field experiment. Four types of organic residues contrasting in chemical quality (i.e., nitrogen (N), lignin, polyphenols, cellulose) were incorporated into soil annually to assess decomposition and associated meso- and macrofauna communities. In the first 4 weeks after residue incorporation (loop #1), the abundances (densities) of both mesofauna and macrofauna were positively influenced by labile cellulose and N. The mesofauna Collembola and Acari contributed 70-100% and 0-30% to the decomposition, respectively, while the macrofauna beetles and flies contributed 20-90% and 10-66%, respectively. The abundances were highest under groundnut (high N, low lignin) ([1.35 and 0.85 individual number (g dry litter)-1] for mesofauna and macrofauna, respectively). The presence of macrofauna at week 2 led to a mass loss (R2 = 0.67**), indicating that macrofauna preceded mesofauna in degrading residue. In week 8 (transition of loop #2 to #3), only macrofauna (beetles dominated contributing 65%) played an important role in lignin decomposition (R2 = 0.56**), resulting in a mass loss (R2 = 0.52**). In week 52 (loop #4) macrofauna, ants (Formicidae) replaced beetles as the dominant decomposers showing a feedback reaction to availability of protected cellulose. The Formicidans contributed 94% to the decomposition and influenced losses of mass (R2 = 0.36*) and N (R2 = 0.78***). The feedback loop concept provides a more comprehensive "two-sided" view into decomposition, as regulated simultaneously by two factors, than earlier "one-sided" approaches to soil fauna-mediated decomposition.

Nutrient composition of diverse organic residues and their long-term effects on available nutrients in a tropical sandy soil.

Intensive use of sandy soils for agriculture leads to significant land degradation. The application of locally available organic residues can improve soil fertility, particularly in the context of organic farming practices. This research examined nutrient concentrations in locally available organic residues with different biochemical compositions/qualities and investigated the effects of long-term application of these residues on available nutrients, such as P, K, Ca, Mg, Fe, Mn, and Zn, as well as on total organic carbon (TOC) accumulation in tropical sandy soil. A field experiment was conducted in Northeast Thailand, where four local organic residues, groundnut stover (GN), tamarind leaf litter (TM), dipterocarp leaf litter (DP), and rice straw (RS), had been applied annually for 22 years. These organic residues were acidic (pH 3.7-5.8). The macronutrients N, P, and K were present at elevated levels in the high-quality organic residue GN, whereas medium-quality TM and low-quality RS and DP were dominated by the macronutrients Ca and Mg and the micronutrients Fe, Mn, and Zn. The incorporation of organic residues, particularly TM, resulted in the accumulation of TOC. Furthermore, long-term incorporation of TM increased soil pH, whereas incorporation of GN, DP, and RS did not. The higher increase in the soil pH of TM soil is likely because TM contains higher levels of ash alkalinity compared to other residues. The application of medium-quality TM increased the soil available P, Ca, and Mg, whereas low-quality organic residue RS and DP applications increased the concentrations of soil micronutrients (e.g., Mn and Zn). However, long-term applications of local organic residues did not increase available K in the sandy soil.

Biochar Properties Influencing Greenhouse Gas Emissions in Tropical Soils Differing in Texture and Mineralogy.

The ability of biochar applications to alter greenhouse gases (GHGs) (CO, CH, and NO) has been attracting research interest. However, inconsistent published results necessitate further exploration of potential influencing factors, including biochar properties, biochar rates, soil textures and mineralogy, and their interactions. Two short-term laboratory incubations were conducted to evaluate the effects of different biochars: a biochar with low ash (2.4%) and high-volatile matter (VM) (35.8%) contents produced under low-temperature (350°C) traditional kiln and a biochar with high ash (3.9%) and low-VM (14.7%) contents produced with a high-temperature (800°C) Flash Carbonization reactor and different biochar rates (0, 2, and 4% w/w) on the GHG emissions in a loamy-sand Ultisol and a silty-clay-loam Oxisol. In the coarse-textured, low-buffer Ultisol, cumulative CO and CH emissions increased with increasing VM content of biochars; however, CO emission sharply decreased at 83 µg VM g soil. In the fine-textured, high-buffer Oxisol, there were significant positive effects of VM content on cumulative CO emission without suppression effects. Regarding cumulative NO emission, there were significant positive effects in the Mn-rich Oxisol. Ash-induced increases in soil pH had negative effects on all studied GHG emissions. Possible mechanisms include the roles biochar VM played as microbial substrates, a source of toxic compounds and complexing agents reducing the toxicity of soil aluminum and manganese, and the role of biochar ash in increasing soil pH affecting GHG emissions in these two contrasting soils.

Can't see the forest for the rice: factors influencing spatial variations in the density of trees in paddy fields in northeast Thailand.

The widespread presence of trees in paddy fields is a unique feature of Northeast Thailand's agricultural landscape. A survey of spatial variability in the density of trees in paddy fields in the Northeast Region was conducted utilizing high resolution satellite images and found that the mean density in the whole region was 12.1 trees/ha (varying from a high of 44.6 trees/ha to a low of 0.8 trees/ha). In general, tree densities are higher in the southeastern part of the region and much lower in the northern central part. Tree density was influenced by multiple factors including: (1) the history of land development, with more recently developed paddy fields having higher densities, (2) topography, with fields located at higher topographical positions having a higher mean density of trees, (3) access to natural forest resources, with fields in areas located close to natural forests having higher densities, (4) amount of annual rainfall, with fields in areas with higher average annual rainfall having higher tree densities, and (5) landholding size, with fields in areas with larger-sized landholdings having more trees. However, there is a considerable extent of co-variation among these factors. Although trees remain an important element of the paddy field landscape in the Northeast, it appears that their density has been declining in recent years. If this trend continues, then the vast "invisible forest" represented by trees in paddy fields may truly disappear, with negative consequences for the villagers' livelihoods, biodiversity conservation, and carbon sequestration in the rural ecosystem.