Root Organic Matter Research Articles

Stable isotope ratios of nonexchangeable hydrogen in organic matter of soils and plants along a 2100-km climosequence in Argentina: New insights into soil organic matter sources and transformations?

The quantitative contribution of shoot and root organic matter (OM) to the soil carbon (C) stock is still unknown, mainly because of methodological restrictions. The novel measurement of the nonexchangeable hydrogen (H) stable isotope ratio (δ2Hn) in bulk OM provides new opportunities to investigate the sources of soil C and its climate-dependent transformations. Our objectives were to test whether (I) there are systematic differences between δ2Hn values of aboveground and belowground plant OM and (II) δ2Hn values of litter and soil OM relate to climate and plant OM source δ2Hn signals. We determined δ2Hn values of bulk shoot, root, litter and demineralized soil OM from 20 sampling sites along a 2100-km climosequence from the Argentinean Pampas to the Patagonian steppe. The δ2Hn values of shoot and litter OM correlated closely with the aridity index (r=−0.83, p<0.001 and r=−0.78, p<0.001, respectively) because of H isotope fractionation during aridity-controlled transpiration in shoots. In contrast, δ2Hn values of root and soil OM showed a close correlation with modeled mean annual δ2H values of local precipitation (r=0.91, p<0.001 and r=0.97, p<0.001, respectively, root mean square error of 8.2‰ and 7.2‰, respectively) and thus mean annual temperature (r=0.80, p<0.001 and r=0.88, p<0.001, respectively). δ2Hn values of shoot and root OM differed markedly (no linear correlation) most likely because of biosynthetic exchange of C-bound H with ambient water in the roots. δ2Hn values of root and demineralized soil OM, however, were closely correlated (r=0.91, p<0.001) with a constant offset irrespective of climatic conditions, suggesting that root OM was a more important source of soil OM than shoot OM. A possible contribution of shoot OM to soil OM could only be explained if shoot OM underwent biosynthetic exchange of C-bound H with ambient water in soil during microbial and fungal decomposition. This mechanism is known for substrates processed through the microbial and fungal glycolysis–gluconeogenesis metabolic pathways. Our modeling suggested that the δ2Hn signature of soil OM is best explained under the assumption that root OM is the predominant source of soil OM, rather than shoot and litter OM.

Opposite carbon isotope discrimination during dark respiration in leaves versus roots – a review

In general, leaves are (13) C-depleted compared with all other organs (e.g. roots, stem/trunk and fruits). Different hypotheses are formulated in the literature to explain this difference. One of these states that CO2 respired by leaves in the dark is (13) C-enriched compared with leaf organic matter, while it is (13) C-depleted in the case of root respiration. The opposite respiratory fractionation between leaves and roots was invoked as an explanation for the widespread between-organ isotopic differences. After summarizing the basics of photosynthetic and post-photosynthetic discrimination, we mainly review the recent findings on the isotopic composition of CO2 respired by leaves (autotrophic organs) and roots (heterotrophic organs) compared with respective plant material (i.e. apparent respiratory fractionation) as well as its metabolic origin. The potential impact of such fractionation on the isotopic signal of organic matter (OM) is discussed. Some perspectives for future studies are also proposed .

The effect of grazing frequency and intensity on the root development of Lolium perenne L. under subtropical conditions

The root organic matter (OM) per unit volume of soil of Lolium perenne L. (perennial ryegrass) was evaluated over two years under subtropical conditions. Root OM was measured on five separate occasions in four soil-depth categories (0–50, 50–100, 100–200 and 0–200 mm) under six combinations of grazing frequency and intensity. The root systems were unaffected by treatments during the establishment year. During the second year, treatments infrequently grazed had higher root OM contents per unit volume of soil in both the 0-50- and 0-200-mm soil-depth categories than treatments frequently grazed. Grazing intensity did not affect root OM per unit volume of soil in any depth class. Overall, perennial ryegrass was found to be shallow-rooted, with approximately 75% of the root mass occurring in the top 50, mm of the soil.

Research note: The influence of applied nitrogen on the root development ofLolium perenneL. one year after establishment

Abstract The effect of different levels of applied nitrogen (N) (ranging from 120–720 kg N ha−1 a−1) on the root development of perennial ryegrass (Lolium perenne L.) was examined at the end of the establishment year. Increasing levels of applied N increased perennial ryegrass root organic matter (OM) content in the top 50 mm of the soil, while a corresponding decrease in root OM content was observed in the 100–200 mm soil depth category. Increasing levels of applied N did not have a significant effect on root OM content in the 50–100 mm or the overall 0–200 mm soil depth category. Lower levels of applied N (120–360 kg N ha−1 a−1) may be more conducive to promoting deeper root development than higher levels of applied N (480–720 kg N ha−1 a−1), but this would require investigation beyond one year. Perennial ryegrass is extremely shallow‐rooted, with approximately 75 % of the root mass occurring in the top 50 mm of the soil. This stresses the need to pay careful attention to irrigation scheduling.