Dissimilar evolution of soil dissolved organic matter chemical properties during revegetation with arbor and shrub in desertified land of the Mu Us Desert

The chemistry of dissolved organic matter (DOM) in soil determines its bioavailability and is crucial to soil nutrient cycling and vegetation restoration. However, the response of DOM to revegetation in degraded ecosystems is not well understood. This study analyzed soil chronosequences from semifixed (SF) sand in the Mu Us Desert, China that was afforested with arbor and shrub 23-54 years ago. The DOM organic composition and chemical properties were investigated by fluorescence excitation-emission matrix spectrophotometry and UV-visible spectroscopy. As the revegetation progressed, DOM concentration at 0-20 cm continually increased. In arbor land, the DOM was mainly influenced by dissolved organic carbon, and the average concentration significantly increased from 67.5 to 203.2 mg C.kg(-1). In the shrubland, changes in DOM were mainly influenced by dissolved organic nitrogen, with the average concentration significantly increasing from 3.7 to 8.9 mg N.kg(-1). Arbor afforestation significantly increased the proportion of hydrophobic humic-like components (C1), with a decline in bioactive tryptophan-like components (C3) and higher DOM molecular complexity (i.e., higher molecular weight, aromaticity, and degree of humification). However, in shrubland, the proportions of C1 and C3 were significantly lower than those in arbor forest soil, with DOM molecular complexity slightly increased, and DOM tended to become more bioavailable over time. Linear regression analysis indicated that DOM rich in carbon had a high molecular complexity, while DOM rich in nitrogen is more bioavailable. These findings reveal a dissimilar evolution in the DOM composition and chemistry in the two forest types throughout the SF sand revegetation period. The DOM under arbor forest was more stable and conducive to soil organic matter sequestration, while the DOM under shrub forest contained more bioactive substances and was more favorable for nutrient cycling.