Soil fertility, microbial communities, and nitrogen dynamics along rainfall gradients
Several well-defined pedogenic thresholds, where soil properties relating to weathering and leaching (Al and Fe pools, base saturation, pH, exchangeable calcium, total and resin P) change abruptly along a continuous gradient in environmental forcing, have been identified along a rainfall gradient from 280 – 3400 mm/yr in Hawaii. Here, we asked how those thresholds and the domains between them mapped onto the biological availability of N – likely the nutrient that exerts the strongest limitation on plant production along the gradient. In particular, there is a domain receiving rainfall of ~900 to ~1800 mm/yr where ongoing weathering and biological uplift create a “sweet spot” of soils rich in available P and Ca; we asked if that domain is also rich in available N, and if so why? To answer these questions, we collected soil from 46 sites along the gradient and measured net and gross N mineralization at ambient and adjusted soil moisture. We also analyzed microbial community composition along the gradient by sequencing ribosomal marker genes for bacteria + archaea (16S) and fungi (ITS) using Illumina MiSeq, and characterized functional genes associated with N-cycling using quantitative PCR and cloning.
Rates of potential net N mineralization and nitrification (in lab incubations, adjusted to constant water content) averaged 1-2 mg kg-1 day-1 in the “sweet spot” soil domain, and < 0.5 mg kg-1 day-1 in wetter and drier sites. Peak net mineralization and nitrification at ambient (field) soil moisture occurred at ~1500 mm/yr, below which mineralization was water-limited even within the “sweet spot”. The first axis of NMDS ordinations of microbial community composition correlated with the overall rainfall gradient (and with soil pH) for bacteria+archaea and for fungi, while the second axis for both groups correlated with high levels of available Ca and P, potential N mineralization, and the “sweet spot”. These correlations were driven by changes in the relative abundance of major microbial lineages including Acidobacteria and Ascomycota. Sequences corresponding to the nitrifying archaea Candidatus Nitrososphaerawere most abundant at ~1000 mm/yr; we observed a peak in archaeal ammonia monooxygenase gene copy number (by qPCR) there as well. Soil properties related to weathering and leaching apparently affect N transformations and microbial communities substantially; determining the pathways by which they operate requires further research.