The role of arbuscular mycorrhizal colonization in the carbon and nutrient economy of the tripartite symbiosis with nodulated Phaseolus vulgaris

In the tripartite symbiosis between nodulated legume roots and arbuscular mycorrhizal (AM) fungi, symbiont sink strength may depend upon developmental stage and the nutrient benefits to the host plant. The cost-benefits of the tripartite symbiosis were investigated in terms of C-economy and nutrition. Nodulated Phaseolus vulgaris seedlings, with and without AM, were hydroponically grown under high (2 mM) and low (1 μM) P conditions in an N-free Long Ashton nutrient solution. Plants were sequentially harvested at 17, 24 and 31 days after emergence. At each harvest, measurements for biomass, N₂-fixation, photosynthesis, root respiration, calculated C and nutritional economy were taken. Nodular growth was suppressed by the early development of AM colonization. This coincided with higher photosynthetic and respiratory rates in AM plants. These effects were most pronounced under low P when AM colonization peaked. Once AM levels reached the plateau phase, the efficiency of P nutrition increased. This was followed by improved nodular and host growth and enhanced N₂-fixation. This indicates that the AM was the dominant symbiont for host C in the tripartite symbiosis, due to its rapid development and subsequent role in supplying P more effectively to both host and nodules.     

Soil organic N - An under-rated player for C sequestration in soils?

The availability of Soil Organic Nitrogen (SON) determines soil fertility and biomass production to a great extent. SON also affects the amounts and turnover rates of the soil organic carbon (SOC) pools. Although there is increasing awareness of the impact of the nitrogen (N) cycle on the carbon (C) cycle, the extent of this interaction and the implications for soil organic matter (SOM) dynamics are still under debate. Therefore, present knowledge about the inter-relationships of the soil cycles of C and N as well as current ideas about SON stabilization are summarized in this paper in order to develop an advanced concept of the role of N on C sequestration. Modeling global C-cycling, it was already recognized that SON and SOC are closely coupled via biomass production and degradation. However, the narrow C/N ratio of mature soil organic matter (SOM) shows further that the impact of SON on the refractory SOM is beyond that of determining the size of the active cycling entities. It affects the quantity of the slow cycling pool and as a major contributor it also determines its chemical composition. Although the chemical nature of SON is still not very well understood, both improved classical wet chemical analyses and modern spectroscopic techniques provide increasing evidence that almost the entire organic N in fire-unaffected soils is bound in peptide-like compounds and to a lesser extent in amino sugars. This clearly points to the conclusion, that such compounds have greater importance for SOM formation than previously assumed. Based on published papers, I suggest that peptides even have a key function in the C-sequestration process. Although the mechanisms involved in their medium and long-term stabilization are far from understood, the immobilization of these biomolecules seems to determine the chemistry and functionality of the slow cycling SOM fraction and even the potential of a soil to act as a C sink. Pyrogenic organic N, which derives mostly from incomplete combustion of plant and litter peptides is another under-rated player in soil organic matter preservation. In fire-prone regions, its formation represents a major N stabilization mechanism, leading to the accumulation of heterocyclic aromatic N, the stability of which is still not elaborated. The concept of peptide-like compounds as a key in SOM-sequestration implies that for an improved evaluation of the potential of soils as C-sinks our research focus as to be directed to a better understanding of their chemistry and of the mechanisms which are responsible for their resistance against biochemical degradation in soils.