Abstract
IntroductionThe availability of essential nutrients, such as nitrogen (N) and phosphorus (P), can feedback on soil carbon (C) and the soil microbial biomass. Natural cycles can be supplemented by agricultural fertiliser addition, and we determined whether the stoichiometry and nutrient limitation of the microbial biomass could be affected by an unbalanced nutrient supply.MethodsSamples were taken from a long-term trial (in effect since 1968) with annual applications of 0, 15 and 30 kg P ha−1 with constant N and potassium. Soil and microbial biomass CNP contents were measured and nutrient limitation assessed by substrate-induced respiration. Linear regression and discriminant analyses were used to identify the variables explaining nutrient limitation.ResultsSoil and biomass CNP increased with increasing P fertiliser, and there was a significant, positive, correlation between microbial biomass P and biomass C, apart from at the highest level of P fertilisation when the microbial biomass was over-saturated with P. The molar ratios of C:N:P in the microbial biomass remained constant (homeostatic) despite large changes in the soil nutrient ratios. Microbial growth was generally limited by C and N, except in soil with no added P when C and P were the main limiting nutrients. C, N and P, however, did not explain all the growth limitation on the soils with no added P.ConclusionsIncreased soil C and N were probably due to increased net primary production. Our results confirm that C:N:P ratios within the microbial biomass were constrained (i.e. homeostatic) under near optimum soil conditions. Soils with no added P were characterised by strong microbial P limitation and soils under high P by over-saturation of microorganisms with P. Relative changes in biomass C:P can be indicative of nutrient limitation within a site.
Highlights
The availability of essential nutrients, such as nitrogen (N) and phosphorus (P), can feedback on soil carbon (C) and the soil microbial biomass
We found only a weak correlation between the chloroform fumigation-extraction (CFE) and substrate-induced respiration (SIR) measurements of Cmic, and variation was large in the high P-fertiliser treatments (15_15, 30_0, 30_30)
Our results confirm that C:N:P ratios within the microbial biomass were constrained as long as soil conditions were not far away from the optimum
Summary
The availability of essential nutrients, such as nitrogen (N) and phosphorus (P), can feedback on soil carbon (C) and the soil microbial biomass. In terrestrial ecosystems most primary production enters the decomposer pathway (Cebrian 1999), where microorganisms mineralise organic material to simple inorganic compounds and recycle growth-limiting nutrients for autotrophs. This is essential for soil fertility and plant growth. In a global-scale meta-analysis of the C:N:P ratio of soil and the soil microbial biomass (Cleveland and Liptzin 2007), the abundance and ratio of elements were constrained While this was not surprising for C and N, given that plants are the major source of both C and N in the soil, relatively fixed C:P and N:P ratios were more unexpected given that organisms do not regulate the total amount of soil P. In managed systems nutrient addition is commonly practised, and even in unmanaged systems nutrient deposition occurs, so the objective of this study was to determine how soil and soil microbial biomass elemental ratios responded to nutrient addition
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