Litter inputs and phosphatase activity affect the temporal variability of organic phosphorus in a tropical forest soil in the Central Amazon

Abstract

Purpose The tropical phosphorus cycle and its relation to soil phosphorus (P) availability are a major uncertainty in projections of forest productivity. In highly weathered soils with low P concentrations, plant and microbial communities depend on abiotic and biotic processes to acquire P. We explored the seasonality and relative importance of drivers controlling the fluctuation of common P pools via processes such as litter production and decomposition, and soil phosphatase activity. Methods We analyzed intra-annual variation of tropical soil phosphorus pools using a modified Hedley sequential fractionation scheme. In addition, we measured litterfall, the mobilization of P from litter and soil extracellular phosphatase enzyme activity and tested their relation to fluctuations in P- fractions. Results Our results showed clear patterns of seasonal variability of soil P fractions during the year. We found that modeled P released during litter decomposition was positively related to change in organic P fractions, while net change in organic P fractions was negatively related to phosphatase activities in the top 5 cm. Conclusion We conclude that input of P by litter decomposition and potential soil extracellular phosphatase activity are the two main factors related to seasonal soil P fluctuations, and therefore the P economy in P impoverished soils. Organic soil P followed a clear seasonal pattern, indicating tight cycling of the nutrient, while reinforcing the importance of studying soil P as an integrated dynamic system in a tropical forest context.

Extracellular enzyme activities in tropical soils are driven by seasonal litter input

“…Our data shows a chronic high investment in AP, suggesting the system to be P limited as hypothesized. Phosphorus, when it has limited bioavailability in soil, must be cycled tightly, warranting both enzymatic production when available P is low and strong linkage between the enzyme activities and litter inputs to the system (Schaap et al 2021). Our EE data is in the same range as compared to enzyme activities measured along an altitude gradient in the Andes, albeit lower than on lower altitudes reported (Nottingham et ), but in the referred studies annual temperature variations were larger.…”

“…Total soil C and N were determined in milled dry aliquots by mass spectrometry (Finnegan Delta Plus) for elemental C and N (Carlo-Erba, 1110). Total P was determined in dry (unmilled) 0.5 g aliquots with the molybdate blue method (Murphy and Riley 1962) after acid digestion using concentrated sulphuric acid solution (H 2 SO 4 , 18 M), followed by H 2 O 2 (Quesada et al 2010; see also Schaap et al 2021).…”

“…2013;Cotrufo et al 2015). This time lag is a possible explanation for the vectors to show a relative trend towards more P-acquisition towards the end of the drier season; P-loss from litter is not immediate(Martins et al 2021) and this might cause this delay in enzymatic response(Schaap et al 2021). In both the drier and the wetter season, the vector angles are indicating a rather strong indication of P-limitation(Moorhead et al 2016), especially at the lower depth.…”

Soil phosphorus fractions and their relation to leaf litterfall in a central Amazonian terra firme rainforest

“…action (Richardson et al 2005). Since plants can only absorb inorganic orthophosphate (Raghothama 2005), mobilizing P o forms first involves the liberation of P o from precipitates and adsorption sites, followed by the mineralization of these P o forms into plant available P i through the action of phosphatase enzymes (Richardson et al 2005;Clarholm et al 2015;Schaap et al 2021). However, P i -NaOH can also be an important source of inorganic P in the P cycle, and may transfer a significant amount of P to other fractions or be taken up by plant roots in the form as orthophosphates.…”

“…Inorganic and organic phosphorus levels in forest soils are expected to change across seasons (Fabre et al 1996;Chen et al 2003;Schaap et al 2021). Concordantly, we found a clear and significant temporal variation in some P fractions over the five months of study, in particular the inorganic and organic P-NaOH forms and P-HCl.…”