Abstract

Plant litter is one of the main sources of soil humus, but which can also promote primary humus degradation by increasing microbial activity due to the higher availability of energy released, resulting in a confusing relationship between litterfall and soil humus. Therefore, an in situ incubation experiment was carried out in three subalpine forests (coniferous, mixed and broadleaved forests) on the eastern Qinghai-Tibetan Plateau. We set up two treatments. One that allowed litterfall to enter the soil normally and the other prevented litterfall to enter the soil. Soils were sampled in October (the end of the growing season), January (the onset of the freezing season), March (the end of the freezing season), and May (the start of the growing season) from May 2017 to May 2018. By assessing the litterfall production, the content of total extracted humus, humic acid (HA) and fulvic acid (FA) in the topsoil (0–20 cm) in each incubation period, we determined the impact of litterfall on the content of humus extracted from the soil during the freezing and the growing season. Over 1-year incubation, soil total extracted humus and HA showed considerable decreases in the treatment of retained litterfall in the mixed forest but not in the coniferous or broadleaved forests. Moreover, litterfall significantly reduced the contents of soil total extracted humus and HA during the growing season in all three forests, while only reduced soil HA content in the broadleaved forest in the freezing season. The relationship between litterfall and soil extracted humic substances was greatly regulated by the seasonal dynamics of litter types and litter production in all forest types. The larger the amount of litterfall was, the more litterfall could promote the reduction of soil extracted humic substances. Compared with a single type of broadleaf or needle litter, mixed litterfall could promote a higher degradation of soil humic substances. However, broadleaf litter might lead to much greater decreases in soil humic substance than needle litter because it is more decomposable. These results indicate that the effect of litterfall on soil humic substances are mainly regulated by litter types and litter production. Moreover, the effects of litterfall on soil humic substances are more significant during the growing season than winter. This suggests that the longer growing season and a shorter winter caused by ongoing global warming may alter the relationships between litterfall and extracted humic substances, further disrupting the carbon balance of forest ecosystems in the subalpine forests.

Highlights

  • Humus is the main component of soil organic matter (KogelKnabner, 2000) and is important for soil fertility and nutrient cycling (Ponge, 1999; Ono et al, 2011; Chertov and Komarov, 2013)

  • We found that soil humic acid (HA) content and HA/fulvic acid (FA) ratio significantly decreased in mixed forests after 1 year of continuous litterfall input, but FA content was not affected by litter input

  • Litterfall significantly decreased the soil humic substances and HA content of three forests during the growing season but showed insignificant effects in freezing season, implying that a longer growing season and shorter winter caused by global warming may promote the degradation of soil humic substances and the potential loss of soil organic matter

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Summary

Introduction

Humus is the main component of soil organic matter (KogelKnabner, 2000) and is important for soil fertility and nutrient cycling (Ponge, 1999; Ono et al, 2011; Chertov and Komarov, 2013). There is evidence that shows that the incorporation of fresh organic matter into soil can increase microbial activity due to the higher availability of energy released, exacerbate soil humus mineralization as named “priming effect” (Broadbent and Nakashima, 1974; Wu et al, 1993; Liljeroth et al, 1994). As such, it is still unclear whether the input of litterfall promotes the synthesis or degradation of soil humic substances. Both climatic factors (Prescott et al, 2000; Ponge, 2013) and forest litter type (Langenbruch et al, 2012) significantly affect litter humification and soil humus development, which further complicates the relationship between litterfall and soil humic substances

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