Seasonal changes in soil organic matter after a decade of nutrient addition in a lowland tropical forest

AbstractSoil 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.

Download Full-text

2nd Annual Report: Tropical Forest Response To A Drier Future: Turnover Times Of Soil Organic Matter, Roots, Respired CO2, And CH4 Across Moisture Gradients In Time And Space

10.2172/1466169 ◽

2018 ◽

Author(s):

Karis J. McFarlane

Keyword(s):

Organic Matter ◽

Soil Organic Matter ◽

Tropical Forest ◽

Annual Report ◽

Time And Space

Download Full-text

Tropical forest soil carbon stocks do not increase despite 15 years of doubled litter inputs

Scientific Reports ◽

10.1038/s41598-019-54487-2 ◽

2019 ◽

Vol 9 (1) ◽

Cited By ~ 5

Author(s):

Emma J. Sayer ◽

Luis Lopez-Sangil ◽

John A. Crawford ◽

Laëtitia M. Bréchet ◽

Ali J. Birkett ◽

...

Keyword(s):

Organic Matter ◽

Tropical Forest ◽

Critical Role ◽

Tropical Soils ◽

Soil Geochemistry ◽

Soil Minerals ◽

Litter Addition ◽

Lowland Tropical Forest ◽

Tropical Forest Soil ◽

Underlying Mechanisms

AbstractSoil organic carbon (SOC) dynamics represent a persisting uncertainty in our understanding of the global carbon cycle. SOC storage is strongly linked to plant inputs via the formation of soil organic matter, but soil geochemistry also plays a critical role. In tropical soils with rapid SOC turnover, the association of organic matter with soil minerals is particularly important for stabilising SOC but projected increases in tropical forest productivity could trigger feedbacks that stimulate the release of stored SOC. Here, we demonstrate limited additional SOC storage after 13–15 years of experimentally doubled aboveground litter inputs in a lowland tropical forest. We combined biological, physical, and chemical methods to characterise SOC along a gradient of bioavailability. After 13 years of monthly litter addition treatments, most of the additional SOC was readily bioavailable and we observed no increase in mineral-associated SOC. Importantly, SOC with weak association to soil minerals declined in response to long-term litter addition, suggesting that increased plant inputs could modify the formation of organo-mineral complexes in tropical soils. Hence, we demonstrate the limited capacity of tropical soils to sequester additional C inputs and provide insights into potential underlying mechanisms.

Download Full-text