The use of organic and composted residues as a source of nutrients is increasingly important in sustainable management techniques. Compost has a great potential to recycle carbon and nutrients that otherwise would have to be disposed of, to improve the energy efficiency of the cropping systems, and to contribute to sequestration of soil C. Replacing mineral N fertilizers with compost could increase the environmental sustainability of fertilizers and the agro-ecosystem. However, the ability of compost to completely sustain crop N uptake through mineralization needs to be confirmed.

The quantification of nitrogen cycling in agriculture requires the use of special techniques, especially to measure the recovery of N fertilizers in soil. Stable isotopes have been used to trace the fate and movement of nutrients in the environment and to quantify the fractional contribution of soil organic matter (SOM) and added organic material to plant uptake. The calculation of N losses from crop-soil systems can successfully be done using 15N-labelled fertilizer (Powlson et al. 1992). It has been shown that N uptake by crops from organic inputs such as plant residues or manures is often less than 20%. However, although this amount seems little, it has been widely accepted that organic inputs play a significant role in the long-term build-up of organic matter in soil and its stabilization. The use of 15N in organic fertilizer studies has significantly advanced our understanding of N release from organic materials. 15N direct labelling techniques are used to study crop residue or green manure contribution to plant nutrition. Green manures can be easily obtained by growing crops fertilized with 15N tracer. The above-or belowground material is then harvested and added as residues to unlabelled soil where the next crop is grown. This crop is then harvested and the percentage nitrogen in the crop derived from added residues can be calculated (Hauck and Bremner 1976).

Indirect techniques have also been used to study plant N uptake from organic residues. 15N tracer is added to soil, and treatments with and without residues (no-residue controls) are set up. The no-residue controls should have an 15N enrichment that reflects the joint soil plus 15N-N fertilizer pool, while the residue treatments should have a lower 15N enrichment due to input of unlabelled N originating from the decomposing residue. The nitrogen derived from residue (Ndfr) can then be calculated. This is the same principle used in 15N dilution methods to estimate the biological nitrogen fixation.

Measurements of natural 15N abundance (d15N) in organic materials and in soil particles can be used to characterize their origin and the rate of N changes involved. The variation in natural abundance as 15N/14N ratio has been used in fewer studies (e.g., Kerley and Jarvis 1997) with respect to artificially enriched materials. One reason is that it is quite difficult to identify well-defined natural N-pools of considerably different isotopic composition (Gerzabek et al. 2001). This is in contrast to C, where material from C3 and C4 plants can be used as tracers due to their significant differences in 13C/12C ratios. Variation in natural abundance of

N stable isotopes in soil is explained by both (1) the input of N from different sources with different isotopic compositions and (2) isotopic fractionations during transport and especially microbial turnover (Macko and Estep 1984). Fractionation in the soil N cycle complicates the quantification of N turnover and dynamics derived from typical agricultural N sources using natural abundance (15N/14N ratio), although in some studies involving reference treatments the quantification of source-derived N was partly successful (e.g., Wagner 1991).

Studies on the decomposition and mineralization of labelled plant residues have commonly been conducted under well-defined greenhouse or laboratory conditions (Cortez and Hameed 2001; Semenov et al. 2001). In addition, various field studies have observed the decomposition of labelled organic matter in agricultural croplands. For example, Voroney et al. (1989) monitored field decomposition of labelled wheat straw in the short- (2 years) and long-term (up to 10 years) in the fields using bulk soil material without any soil fractionation.

Researchers used N isotopes to track nutrient partitioning among different soil constituents. Tiessen et al. (1984) showed that sand and silt fractions had a low enrichment, whereas clays had a high enrichment. They related this difference to an association of low enrichment plant material with larger particles. In addition, because clay provides N transformation sites and is physically associated with organic materials, there is an enrichment in this fraction.

Aita et al. (1997) and Haynes and Beare (1997) analysed the short-term (2 years) decomposition of labelled plant residues by separating soil into particle-size fractions. Within the first year of the study, the authors showed a rapid decrease of 15N in coarse and light fractions that represent decomposing plant residue material. As a consequence, a concomitant 15N enrichment in fine particle-size fractions (<50 mm) was observed, including microbial derived organic constituents (Haynes and Beare 1997). Swanston and Myrold (1997) showed that after 21 months of incubation only one-third of the 15N recovered from labelled red alder leaves was found in the heavy fractions.

Gerzabek et al. (2001) studied the nitrogen distribution and 15N natural abundance in particle size fractions in a long-term agricultural field experiment. They reported that in most cases d15N values increased with decreasing particle size. They also concluded that the natural abundance of 15N in bulk soil and particle size fractions was significantly altered by the long-term application of Ca-nitrate and organic manures. Kolbl et al. (2006) analysed the short-term (570 days) decomposition of labelled mustard litter by separating soil into particle-size fractions. After 570 days of application of 15N-mustard litter to an agricultural cropland, the distribution of 15N was measured in particulate organic matter (POM) fractions and in fine mineral fractions (fine silt- and clay-sized fractions). After 570 days, only 2.5% of the initial 15N amount was found in POM fractions, with larger amounts in aggregate-occluded POM than in free POM. After this period, stabilization of initial 15N in fine silt- and clay-sized fractions amounted to 10% in high-yield soils, but 20% in low-yield soils, while 70-85% of the added 15N was lost. They also found that in some croplands up to 25% of applied 15N was stabilized in clay-sized fractions after 161 days.

Mueller et al. (2009) found that the POM fractions in 0-2 cm of the topmost soil layer showed decreasing 15N concentrations in the free particles and increasing concentrations in the occluded particles. Swanston and Myrold (1997) also observed the highest 15N recovery in <5 cm of the topmost soil layers and the light fraction, while below a 5-cm depth, the 15N recoveries were low and variable. In a decomposition study using 15N-labelled beech litter, Zeller et al. (2000) found that 62% of released N came from the surface soil after 3 years but only 12% came from a depth below 2 cm.

The annual input of organic materials to the soil from crops is an important component in the study of organic matter (Balesdent and Balabane 1992). Studying C sequestration, C budget and C dynamics entails quantifying the organic carbon (C) input derived from crop roots to cultivated soil (Hobbie et al. 2002).

Roots play a dominant role in C and N soil cycles (Gale et al. 2000a; Puget and Drinkwater 2001) and may have a relatively greater influence on soil organic C and N levels than the aboveground plant biomass (Boone 1994; Norby and Cotrufo 1998). Root-derived sources of soil organic C (SOC) are the materials released from roots during growth: mucilage, sloughed off root tips, root exudates, gradual loss of cells by the fully functional roots and decaying roots (van Noordwijk et al. 1994).

Amounts of root-derived C or root biomass vary according to environmental conditions, management systems, crop genotypes, and the physical, chemical and biological properties of the soil (Balesdent and Balabane 1992). Root development is also particularly sensitive to variations in the supply and distribution of inorganic nutrients and water (Steingrobe et al. 2001). More importantly, root mucilage, root exudates and detached root tips contribute considerably to total SOC input in the soil (Crawford et al. 1996), which is difficult to measure by conventional methods.

Helal and Sauerbeck (1987) estimated that the amount of C released from plants as rhizodeposition could be more than 580 kg C ha-1. This amount increases microbial activity and influences N mineralization in the soil (Bakken 1990; Texier and Biles 1990). As much as 7-43% of the total above- and belowground plant biomass can be made up of roots (Kuo et al. 1997). Maize and winter cover crop roots can supply from 400 to 1,460 kg C ha-1 during a growing season (Kuo et al. 1997). Balesdent and Balabane (1996) observed that maize (Zea mays L.) roots contributed 1.6 times more C to organic soil C than stover. Root-derived C is retained and forms more stable aggregates than shoot-derived C (Gale et al. 2000a, b). Liang et al. (2002) found that maize roots contributed as much as 12% of soil organic C, 31% of water soluble C and 52% of microbial biomass C within a growing season. The carbon contribution from maize root biomass and rhizodeposition to soil organic C can be as much as 1.7-3.5 times greater than that from stover (Wilts et al. 2004).

Fenandez et al. (2003) studied carbon allocations in a sweet sorghum-soil system using 14C as a tracer in order to assess the contribution of the crop to carbon storage in the soil. Sampling was performed 24 h after labelling and at harvest they concluded that 4-16% of 14C present in the sorghum-soil was located in the soil fine fraction (<2 mm). At the harvest, the proportion of 14C present in the soil accounted for 7-9% of the 14C presented in sorghum-soil system. The plant-derived soil carbon was estimated at 0.10-0.12 g C plant-1 day-1, the carbon total amount captured by sweet sorghum was estimated at 1.44 kg C m-2 over the whole growth cycle: 0.82 kg C m2 in the aboveground biomass, 0.52 kg C m2 in the below ground biomass and 0.10 kg C m2 in the soil carbon pool.

This chapter reviews some of the most useful methodologies and applications of N and C tracers to trace the fate of added organic or mineral material to the soil in natural and agricultural systems. Moreover, the main results from two field experiments are presented: one with N and the other with C isotopes, carried out within the framework of the MESCOSAGR project (see Foreword and Chap. 3). Both experiments were aimed at evaluating the fate of added compost to soil cropped with maize or sorghum. The first quantified the fate and flow rate of N in plant-soil systems, while the second monitored soil C sequestration associated with sorghum plant rhizodeposition.

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