Plant–microbial competition for nitrogen uncoupled from soil C:N ratios

Abstract

A green house experiment was designed to test the idea that competition for inorganic nitrogen (N) between plants and heterotrophic microorganisms occurs in soils with high C:N ratios, qualifying for N limited microbial activity, but not at low C:N ratios. The short-term (24 h) 15 N uptake by the grass Festuca gigantea and microorganisms in planted and unplanted soils was determined, and the bacterial activity was measured by the 3 H-thymidine incorporation technique. Two deciduous forest soils, with C:N-ratios of 20 and 31, and the 20 soil amended with litter to a C:N ratio of 34, were used. A novel and important part of the experimental design was the preparation of the unplanted reference soil with plants present until the competition assay started by the addition of 15 N labelled ammonium (NH 4 ' ) or nitrate (NO 3 ( ). The results suggested that plants and soil microorganisms competed for mineral N but under influence of other factors than the soil C:N ratio. The plants reduced the microbial uptake of NH 4 ' and NO 3 ( in the soil with low C:N ratio, which also had the lowest bacterial activity. The plants had a larger N uptake than microorganisms in the two natural soils but smaller in the litter-amended, and their presence enhanced the bacterial activity, especially in the latter soil. The litteramended soil with its high C:N ratio and easily decomposable C was the soil that best fulfilled the criteria for competition, including a net consumption of mineral N during the assay, the lowest plant uptake of mineral N due to the high N immobilization by microorganisms, and a reduced microbial 15 N uptake-to-bacterial activity in the presence of plants. Thus, other factors, such as the decomposability of the soil C and the bacterial activity, were more important than the soil C:N ratio to the outcome of plantÁmicrobial competition for N.

Non‐additive effects of leaf litter and insect frass mixture on decomposition processes

“…Because leaf litter originally contained a very low level of NH 4 + -N (see Table 1), the non-additive decrease in NH 4 + -N in the mixture treatments would result from a decrease of NH 4 + -N originally presented in insect frass. It is known that soil microbes consume more inorganic N in conditions of abundant available C (Ma˚nsson et al 2009). Therefore, leaf litter and insect frass mixing may increase microbial N immobilization by providing available C that originated from the leaf litter.…”

Plant and microbial uptake of nitrogen and phosphorus affected by drought using 15N and 32P tracers

“…Nitrogen is made available to plants through decomposition of soil organic matter (SOM), but microbes also require N for growth. Plantemicrobe competition for N can be intense when both the demand for N by plants and microbes to grow is high (Dunn et al, 2006;Månsson et al, 2009;Inselsbacher et al, 2010;Xu et al, 2011) affecting plant community composition, ecosystem N retention and carbon (C) sequestration (Zak et al, 1990;van der Heijden et al, 2008;Bloor et al, 2009;Averill et al, 2014).…”

Contrasting effects of ammonium and nitrate inputs on soil CO2 emission in a subtropical coniferous plantation of southern China

“…In theory, plant competition for soil N is greater than soil microbes in the N-limiting ecosystems (Kuzyakov and Xu 2013), but plant-microbial competition for N is uncoupled and opposite in the N saturation ecosystems (Kaye and Hart 1997;Månsson et al 2009). In some grassland and forest ecosystems, low level of N addition increases litter return and stimulates soil microbial activity because it satisfies the N demand of soil microorganisms to decompose the relatively small pool of available labile C substrates with low N contents (Fang et al 2014b); however, high level of inorganic N inputs suppresses the synthesis of lignolytic enzymes by some fungi due to the decrease in organic matter decay extent (Cusack et al 2011;Whittinghill et al 2012).…”