Soil nutrients influence plant productivity and carbon cycling. According to biogeochemical theory, throughout soil development, nitrogen (N) accumulates due to biological nitrogen fixation, whereas rock-derived nutrients, such as phosphorus (P) and cations, decrease due to losses from leaching and weathering processes. In geologically old soils, such as many tropical soils, phosphorus is hypothesized to limit forest productivity and ecosystem processes. In the World’s largest rainforest, the Amazon, only gradient studies have tested this hypothesis, with results demonstrating that wood production rates are highly correlated with total soil phosphorus. However, tree species composition, soil type and climate also change across the Amazon. Thus, large-scale, long-term experiments are necessary to disentangle these factors from the effects of soil nutrients on forest carbon cycling. We test the P-limitation hypothesis in the first large-scale nutrient manipulation experiment in a mature forest in the Central Amazon, at a site representative of the most abundant soil type in Amazonia (Amazon Fertilisation Experiment, AFEX). Since April 2017, we have been applying nitrogen, phosphorus, and a combined cation fertilizer in a replicated factorial design. Here, we report rapid initial responses of key above and belowground ecosystem processes to nutrient addition treatments in the first six months post-fertilization.
Results/Conclusions
Cumulative leaf litterfall production varied from 9.86 to 17.78 Mg ha-1 yr-1 but did not differ significantly among treatments. However, P addition increased the total amount of P in leaf litter inputs during the period of peak litterfall, demonstrating the additions had already increased the amount of P already cycling through the ecosystem (34.8 ± 1.55 g P ha-1 in the treatments receiving P and 27.6 ± 1.77 g P ha-1 in the treatments not receiving P; F1, 27 = 11.58, P < 0.01). Calcium (Ca) and Magnesium (Mg) litterfall inputs also increased in the P and N+P addition treatments (Ca model: F7, 21 = 2.74, P < 0.05; Mg model: F7, 21 = 3.90, P < 0.01). Total soil respiration ranged from 5.04 to 7.41 mmol CO2 m-2 s-1 and was significantly higher in the treatments receiving P compared to treatments without added P, indicating that root and microbial activity was stimulated by P addition (F1,27 = 6.23, P < 0.05). Together, our initial results indicate that phosphorus additions alone and in combination with other nutrients stimulated both belowground carbon fluxes and the amount of nutrients cycling through litter pools in this low fertility tropical forest.