Abstract
Soil organic matter (SOM) in tropical forests is an important store of carbon (C) and nutrients. Although SOM storage could be affected by global changes via altered plant productivity, we know relatively little about SOM stabilisation and turnover in tropical forests compared to temperate systems. Here, we investigated changes in soil C and N within particle size fractions representing particulate organic matter (POM) and mineral-associated organic matter (MAOM) after 13 years of experimental litter removal (L−) and litter addition (L+) treatments in a lowland tropical forest. We hypothesized that reduced nitrogen (N) availability in L− plots would result in N-mining of MAOM, whereas long-term litter addition would increase POM, without altering the C:N ratio of SOM fractions. Overall, SOM-N declined more than SOM-C with litter removal, providing evidence of N-mining in the L− plots, which increased the soil C:N ratio. However, contrary to expectations, the C:N ratio increased most in the largest POM fraction, whereas the C:N ratio of MAOM remained unchanged. We did not observe the expected increases in POM with litter addition, which we attribute to rapid turnover of unprotected SOM. Measurements of ion exchange rates to assess changes in N availability and soil chemistry revealed that litter removal increased the mobility of ammonium-N and aluminium, whereas litter addition increased the mobility of nitrate-N and iron, which could indicate SOM priming in both treatments. Our study suggests that altered litter inputs affect multiple processes contributing to SOM storage and we propose potential mechanisms to inform future work.
Highlights
Tropical forest soils play a crucial role in the global carbon (C) balance: they are the largest natural source of carbon dioxide (CO2) and contain almost 1/3 of global soil C stocks (Stockmann et al 2013)
The decline in total N with litter removal was greater than the decline in Total soil organic C (TOC) and there was a strong trend towards a higher soil C:N ratio in the L- plots (12.5 ± 0.3) relative to the controls (11.1 ± 0.2; F2,12 = 3.8, p = 0.053), but no change in the soil C:N ratio in the L? plots (11.9 ± 0.5; Fig. 1c)
Litter manipulation altered the distribution of C between particulate organic matter (POM) and mineral-associated organic matter (MAOM): a smaller proportion of TOC (79 ± 6%) was contained in MAOM in the L? plots compared to the controls (89 ± 2%; Generalised Linear Models (GLM): (a) v2 = 0.27, p = 0.002) but litter removal did not affect the proportion of TOC contained in MAOM (91 ± 3%)
Summary
Tropical forest soils play a crucial role in the global carbon (C) balance: they are the largest natural source of carbon dioxide (CO2) and contain almost 1/3 of global soil C stocks (Stockmann et al 2013). Even small changes in soil C stocks or turnover rates in tropical forests have the potential to influence atmospheric CO2 concentrations. Global environmental changes such as warming, shifting precipitation patterns, and extreme weather events can influence soil C storage in tropical forests via their impacts on net primary productivity and plant litter inputs to the soil (Sayer et al 2011). Increased plant inputs to the soil should promote SOM formation but experimental studies in temperate and tropical forests have clearly demonstrated that increased inputs of plant litter to the soil accumulate in relatively undecomposed fractions and do not necessarily result in larger pools of stabilised SOM, even over decadal timeframes (Sayer et al 2019; Lajtha et al 2014, 2018; Pisani et al 2016)
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