Expanding the toolbox of nutrient limitation studies: A novel method of soil microbial in‐growth bags to evaluate nutrient demands in tropical forests

Abstract

Ecosystem processes and the organisms involved are generally limited by the availability of one or more elements in soil, an important phenomenon to consider for our understanding of ecosystem functioning and future changes. Especially in tropical forests, typically growing on nutrient depleted soils, nitrogen (N), phosphorus (P) or other limitations are assumed. However, large‐scale nutrient manipulation experiments revealed complex site‐specific patterns and several authors raised the need for novel approaches to reveal deeper mechanistic insights on limitation patterns, especially concerning soil microbial activity. In order to gain such deeper knowledge, based on a short review of previous small‐scale studies focusing on soil micro‐organisms, we developed an experimental approach which controls for common biases, including indirect treatment effects, addition of co‐elements or nutrient pulses. Using this technique, we tested the hypotheses that fungi growing in tropical forest soils are mainly limited by P and that N versus P limitations shift along an altitudinal gradient. Mesh bags of 38 μm filled with sand were amended with weak ion exchange resins loaded with N, P or potassium (K) and buried underneath the litter layer at three altitudinal sites in southern Ecuador. After a period of four months, the in‐growth of fungal hyphae was quantified, phospholipid fatty acids were analysed for a subset of samples, and chemical properties were determined. In line with the first hypothesis, hyphal abundance was increased in P‐amended mesh bags, indicating P limitation. However, this pattern was not significantly shifted along the altitudinal gradient. By contrast, N addition increased hyphal abundance at the lowest site, compared to significant reductions at 2,000 and 3,000 m—not only in fungi but also in bacterial abundance as indicated by PLFA analyses, contradicting common soil‐age hypotheses. Decreased nutrient immobilization and fungal in‐growth at higher elevations suggest slow microbial activity, including nitrification, which may have caused toxic ammonium accumulation. The experimental design offers a promising tool to provide more mechanistic and soil‐focused analyses specifically targeting microbes, which in this system strongly supported the hypothesis of primary fungal P limitation in tropical forest soils. A plain language summary is available for this article.

Nitrogen and phosphorus mineralization and their corresponding monetary values under long‐term integrated soil fertility management practices

“…However, under conditions of low N, microbes stimulate the release of N through accelerating decomposition of organic matter (Dijkstra et al, 2013). Besides, it is also likely that the absence of fertiliser caused nutrient demand that prompted increased soil microbial activity to supply/bridge the limiting nutrient (Camenzind et al, 2019), perhaps explaining the more N mineralised in the no input control than the fertilised systems.…”

Plant-soil-enzyme C-N-P stoichiometry and microbial nutrient limitation responses to plant-soil feedbacks during community succession: A 3-year pot experiment in China

“…In this study, carbon-acquiring enzymes showed greater variation compared to nitrogen-acquiring enzymes from the second to third growth periods ( Supplementary Table 7 ). Meanwhile, the lower SOC content of early- and mid- species in the third growth period may increase the microbial carbon limitation ( Supplementary Figure 5 ; Camenzind et al, 2019 ). However, in late species, the SOC content increased in the third growth period, whereas the soil enzyme activity decreased, possibly because changes in SOC lag behind those of soil microorganisms and thus increase the sensitivity of the soil microbial response to soil nutrients during PSF, particularly in late- species ( Bever et al, 1997 ; Zhang et al, 2021 ).…”

Gradiente altitudinal y su influencia en las características edafoclimáticas de los bosques tropicales

“…En altitudes de 1000 m, 2000 m y 3000 m, en suelos fertilizados con N y P, se obtuvo aumento significativo de hongos por la influencia del P (Camenzind et al, 2015;Camenzind, Scheu y Rillig, 2019); a mayores altitudes, el P aumentó el tamaño de las hifas; comportamiento contrario ocasionó el N (Camenzind et al, 2019). Por otro lado, cuando al suelo se fertiliza con N, P, K y Na, en altitudes de 194 m, 210 m y 3025 m, se obtuvo aumento de hongos en restos de celulosa, por la adición de N, lo que sugiere que la principal limitante para degradación de la celulosa es la falta de N en el suelo (Nottingham et al, 2018).…”