Root-induced nitrogen mineralisation: A nitrogen balance model

Abstract

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of nitrogen (N) to plants by stimulating microbial activity. Two models are described, both of which assume that C released from roots is used by bacteria to mineralise and immobilise soil organic N and that immobilised N released when bacteria are grazed by bacterial-feeding nematodes or protozoa is taken up by the plant. The first model simulates the individual transformations of C and N and indicates that root-induced N mineralisation could supply only up to 10% of the plant's requirement, even if unrealistically ideal conditions are assumed. The other model is based on evidence that about 40% of immobilised N is subsequently taken up by the plant. A small net gain of N by the plant is shown (i.e. the plant takes up more N than it loses through exudation), although with exudate of up to C : N 33:1 less than 6% of the plant's requirement is supplied by root-induced N mineralisation. It is argued, however, that rhizosphere bacteria do not use plant-derived C to mineralise soil organic N to any great extent and that in reality root-induced N mineralisation is even less important than these models indicate.

Physiological shifts in the microbial community drive changes in enzyme activity in a perennial agroecosystem

“…Despite no treatment effect on salt-extractable organic C pools, we also observed a significant correlation between net N mineralization and salt-extractable organic C concentration, suggesting a rapid uptake of root C inputs by microorganisms (McLauchlan and Hobbie 2004;Melillo et al 2002). Some studies have reported a stimulation of net N mineralization by plants attributed to increased microbial activity and subsequent release of inorganic N from grazing on bacteria by protozoa (Clarholm 1985;Franzluebbers et al 1994) or decreased microbial immobilization due to competition for N with plants (Griffiths and Robinson 1992;Jingguo and Bakken 1997). Other studies have documented no effect of plants on net N mineralization (Breland and Bakken 1991;Parkin et al 2002).…”

“…In these cases, turnover rates of the microbial biomass increase but microbial activity associated with decomposition remains stable (Kuzyakov et al 2000). At other times increased microbial activity can lead to faster rates of SOM decomposition (''priming'') and subsequent release of plant available N (Bengtson et al 2012;Dijkstra et al 2009;Jingguo and Bakken 1997;Phillips et al 2010) as a result of increased predation on bacteria or competition for N by plants, for example (Clarholm 1985;Griffiths and Robinson 1992;Kuzyakov et al 2000).…”

Cover Crop Use for Managing Broiler Litter Applied in the Fall

“…Soil NO 3 -N concentrations in spring before planting cotton was increased with increasing broiler litter applications in both cover and no cover treatments (Table 8). Although living plants have the potential to increase net N mineralization (Griffi ths and Robinson, 1992;Dormaar, 1990), the presence of cover crop with no broiler litter application in our study did not signifi cantly increase soil residual NO 3 -N. Th e reduction of soil residual NO 3 -N in the spring for cover crop was attributed to the ability of the winter rye in sequestering soil NO 3 -N as a component of organic compounds in the plant. Likewise, Sainju et al (1998) found lower soil NO 3 -N concentration following a rye cover crop due to its high root density which removes a considerable amount of soil NO 3 -N. In the presence of winter rye cover crop, no signifi cant diff erence in soil nitrate N was obtained between broiler litter at the rates of 4.5 and 9 Mg ha −1 , however, for winter fallow, soil NO 3 -N increased with increasing broiler litter applications, particularly at the highest rate of 13.4 Mg ha −1 (P < 0.001) ( Table 8).…”

Microbial-faunal interactions in the rhizosphere and effects on plant growth

“…However, there is evidence that the microbial loop is insufficient for explaining the protozoan-mediated stimulation of plant growth. By including the amount of nitrogen lost through exudation and modelling N transformations in the rhizosphere, Robinson et al [106] and Griffiths and Robinson [55] concluded that rhizosphere bacteria do not use plant-derived carbon to mineralize soil organic nitrogen to any great extent.…”