Abstract

Mechanistic, multi-compartment decomposition models require that carbon (C) and nitrogen (N) in plant material be distributed among pools of different degradability. For this purpose, measured concentrations of C and N in fractions obtained through stepwise chemical digestion (SCD) and values predicted from near-infrared (NIR) spectra or total plant N concentration were compared. Seventysix cash, forage, green manure and cover crop plant materials representing a wide range in biological origin and chemical quality were incubated in a sandy soil at 15 degrees C and -10 kPa water potential for 217 d. A mechanistic decomposition model was calibrated with data from soil without plant material and initialised by data on amounts of C and N in fractions obtained from SCD directly or C and N in SCD fractions as predicted from NIR spectroscopy or plant N concentration. All model parameters describing C and N flows from plant material were kept at default values as defined in previous, independent works with the same model. When results from SCD were used directly to initialise the decomposition model, C and N mineralisation dynamics were predicted well (r(2) = 0.76 and 0.70 for C mineralisation rates and accumulation of inorganic N, respectively). When a NIR calibration was used to predict the SCD data, this resulted in nearly equally good model performance (r(2) = 0.76 and 0.69 for C and N mineralisation, respectively). This was also the case when SCD data were predicted from plant material N concentration (r(2) = 0.76 and 0.69 for C and N). We conclude that the combined use of a mechanistic decomposition model and quality data from SCD is a highly adequate basis for an a priori description of the mineralisation of both C and N from common agricultural plant materials, and that both NIR spectroscopy and measurement of total N concentration offer good and cost-effective alternatives if they are calibrated with SCD data. (C) 2007 Elsevier Ltd. All rights reserved.

Abstract

In order to improve the basis for utilising nitrogen (N) fixed by white clover (Trifolium repens L.) in northern agriculture, we studied how defoliation stress affected the N contents of major plant organs in late autumn, N losses during the winter and N accumulation in the following spring. Plants were established from stolon cuttings and transplanted to pots that were dug into the field at Apelsvoll Research Centre (60 degrees 42'N, 10 degrees 51' E) and at Holt Research Centre (69 degrees 40' N, 18 degrees 56' E) in spring 2001 and 2002. During the first growing season, the plants were totally stripped of leaves down to the stolon basis, cut at 4 cm height or left undisturbed. The plants were sampled destructively in late autumn, early spring the second year and after 6 weeks of new spring growth. The plant material was sorted into leaves, stolons and roots. Defoliation regime did not influence the total amount of leaf N harvested during and at the end of the first growing season. However, for intensively defoliated plants, the repeated leaf removal and subsequent regrowth occurred at the expense of stolon and root development and resulted in a 61-85% reduction in the total plant N present in late autumn and a 21-59% reduction in total accumulation of plant N (plant N present in autumn + previously harvested leaf N). During the winter, the net N loss from leaf tissue (N not recovered in living nor dead leaves in the spring) ranged from 57% to 74% of the N present in living leaves in the autumn, while N stored in stolons and roots was much better conserved. However, the winter loss of stolon N from severely defoliated plants (19%) was significantly larger than from leniently defoliated (12%) and non-defoliated plants (6%). Moreover, the fraction of stolon N determined as dead in the spring was 63% for severely defoliated as compared to 14% for non-defoliated plants. Accumulation in absolute terms of new leaf N during the spring was highly correlated to total plant N in early spring (R-2 = 0.86), but the growth rates relative to plant N present in early spring were not and, consequently, were similar for all treatments. The amount of inorganic N in the soil after snowmelt and the N uptake in plant root simulator probes (PRS (TM)) during the spring were small, suggesting that microbial immobilisation, leaching and gas emissions may have been important pathways for N lost from plant tissue.