Plant growth is usually constrained by the availability of nutrients, water, or temperature, rather than photosynthetic carbon (C) fixation. Under these conditions leaf growth is curtailed more than C fixation, and the surplus photosynthates are exported from the leaf. In plants limited by nitrogen (N) or phosphorus (P), photosynthates are converted into sugars and secondary metabolites. Some surplus C is translocated to roots and released as root exudates or transferred to root-associated microorganisms. Surplus C is also produced under low moisture availability, low temperature, and high atmospheric CO 2 concentrations, with similar below-ground effects. Many interactions among above-and belowground ecosystem components can be parsimoniously explained by the production, distribution, and release of surplus C under conditions that limit plant growth. What Drives Carbon Allocation in Plants? Highlights Plant growth is normally constrained by nutrients, water or temperature, not photosynthesis, and plants often have surplus carbohydrates. Secondary metabolites are produced in N-limited plants primarily to dispose of surplus carbon, although they may subsequently help reduce browsing damage. Surplus carbohydrates are translocated from leaves and below ground some are discharged via exudates and mycorrhizal fungi. Root exudates contain more of the elements that plants have in surplus, and less of those in short supply. The abundance and type of mycorrhizal fungi is influenced by the amount and composition of surplus carbon in roots. Surplus carbon provides an alternative lens though which to view interactions between plants and soil organisms.

In most terrestrial ecosystems, plant growth is limited by nitrogen and phosphorus. Adding either nutrient to soil usually affects primary production, but their effects can be positive or negative. Here we provide a general stoichiometric framework for interpreting these contrasting effects. First, we identify nitrogen and phosphorus limitations on plants and soil microorganisms using their respective nitrogen to phosphorus critical ratios. Second, we use these ratios to show how soil microorganisms mediate the response of primary production to limiting and non-limiting nutrient addition along a wide gradient of soil nutrient availability. Using a meta-analysis of 51 factorial nitrogen-phosphorus fertilization experiments conducted across multiple ecosystems, we demonstrate that the response of primary production to nitrogen and phosphorus additions is accurately predicted by our stoichiometric framework. The only pattern that could not be predicted by our original framework suggests that nitrogen has not only a structural function in growing organisms, but also a key role in promoting plant and microbial nutrient acquisition. We conclude that this stoichiometric framework offers the most parsimonious way to interpret contrasting and, until now, unresolved responses of primary production to nutrient addition in terrestrial ecosystems.

The flow of photosynthetically fixed C from plants to selected soil C pools was studied after 13 CO 2 pulse labeling of pasture plants under field conditions, dynamics of root-derived C in soil was assessed and turnover times of the soil C pools were estimated. The transport of the fixed C from shoots to the roots and into the soil was very fast. During 27 h, net C belowground allocation reached more than 10% of the fixed C and most of the C was already found in soil. Soil microbial biomass (C MIC ) was the major sink of the fixed C within soil C pools (ca 40-70% of soil 13 C depending on sampling time). Significant amounts of 13 C were also found in other labile soil C pools connected with microbial activity, in soluble organic C and C associated with microbial biomass (hot-water extract from the soil residue after chloroform fumigation-extraction) and the 13 C dynamics of all these pools followed that of the shoots. When the labelling (2 h) finished, the fixed 13 C was exponentially lost from the plant-soil system. The loss had two phases; the first rapid phase corresponded to the immediate respiration of 13 C during the first 24 h and the second slower loss was attributable to the turnover of 13 C assimilated in C MIC . The corresponding turnover times for C MIC were 1.1 days and 3.4 days respectively. Such short turnover times are comparable to those measured by growth kinetics after the substrate amendment in other studies, which indicates that microbial growth in the rhizosphere is probably not limited by substrate availability. Our results further confirmed the main role of the soil microbial community in the transformation of recently fixed C, short turnover time of the easily degradable C in the rhizosphere, and its negligible contribution to more stable soil C storage.

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