Lignin and carbohydrates in soils under secondary forest, alley‐cropping, and continuous farming, Thailand

Abstract

AbstractThis study was designed to investigate the impact of three land‐use practices in Thailand on soil organic matter (SOM) composition. The land uses were continuous farming (till), alley‐cropping, and secondary forest. Samples, taken from the top 10 cm soil surface, were fractionated into clay‐ (< 2 μm), silt‐ (2–20 μm), and sand‐sized particles (20–2000 μm) prior to analyses of C, N, lignin‐derived phenols and individual carbohydrates.As particle size increased, the C/N ratio and the concentration of lignin‐derived phenols increased, whereas the degree of lignin oxidation as well as ratios of microbially derived hexoses to plant‐derived pentoses decreased. Thus, the coarser the particle size, the less the SOM was altered by microbes. Seven years of alley‐cropping increased SOM levels only little but considerably affected SOM composition by affecting the SOM of the sand fraction. The SOM concentration in the sand fraction increased in the order continuous farming < alley‐cropping < secondary forest. In the same order, microbial alteration of lignin and carbohydrates of sand‐sized SOM decreased. Both the different concentrations of SOM in the sand fractions as well as its different composition contributed, therefore, to different SOM among the sites. Increasing lignin‐to‐carbohydrate ratios for bulk soils and fractions in the order continuous farming < alley‐cropping < secondary forest reflected such changes in SOM composition sensitively.