The 15N tracer technique can be applied to evaluate both the net/gross mineralization rates of indigenous or added N, and the efficiency of plants to assimilate added N (Barraclough 1995). If the objective is to estimate the plant assimilation of added N, it is usually not convenient to use N tracers, and a lower cost alternative is to apply non-isotopic techniques (Powlson and Barraclough 1993). There are two methods:
(a) Control plot method: N uptake of the unfertilized crop is subtracted from that of the N fertilized crop. This method has the disadvantage that the N uptake of a crop that has not received N is fundamental in the calculation. A bias could thus occur as it is assumed that the N uptake from soil is the same in both unfertilized and fertilized crops (Powlson and Barraclough 1993; Wivstad 1999).
(b) Regression method: the uptake efficiency of fertilizer N is calculated as the slop of regression curve for crop N uptake when several different fertilizer N rates are applied (Wivstad 1999).
In the following sections, estimations of the gross mineralization and nitrification rates as well as the net mineralization and plant assimilations are reviewed.
22.214.171.124 Gross Mineralization and Nitrification Rates
Rates of gross N mineralization and nitrification can be estimated by using an 15N tracer based on two different approaches. The first (analytical method) refers to the isotope dilution technique and analytical equations that relate changes in 15N abundance of a labelled ammonium or nitrate pool to the rate of mineralization or nitrification for short-term estimates. The second (numerical method) refers to simulation models of the soil-plant nitrogen cycle and uses 15N data (such as 15N
abundance in soil mineral N), being preferable for long-term evaluations (Rutting and Muller 2007). The pool dilution method (isotope dilution) is based on a first-order decay equation modelling the decrease of 15N concentration in NH4+ (gross mineralization) or NO3— (gross nitrification) pool, due to concurrent dilution with newly produced NH4+ or NO3— and input in other pools (Perelo et al. 2006). This method has been applied to different studies, including crop residue mineralization and their interaction with soil organic N turnovers (Watkins and Barraclough 1996).
Net rates of mineralization and plant assimilation can be estimated by monitoring 15N abundance in sources and in the final sink. Despite the unique possibility of tracing N in all the studied systems, some problems arise with the nitrogen added to soil and the need to determine 15N abundance of plant available N. This latter problem can be solved by growing the same plant in a non-fertilized control experiments, as illustrated for the case of reference plants in N2-fixation studies. The former problem has often been considered null (Giusquiani et al. 1994).
Different equations have been developed as functions of research assumptions. The same approach can be used to calculate the fractional contribution of an N source (reported here as labelled fertilizer) to total N content in either a plant (reported here as crop) or a soil (in substitution of the plant N content).
The following are the most commonly used equations presented in the order of increasing simplicity of application and increasing assumptions required:
1. Equation (5.1) takes into account both soil 15N abundance and isotopic fraction-ation during plant uptake of soil and fertilizer N. It can be used both in the studies dealing with natural abundance (d15N) and 15N-enriched fertilizer (atom% 15N):
%N from fertilizer d15Nnon—labelled crop — d15N—labelled crop
d15N non - labelled crop — d15N crop grown only on labelled fertilizer
It is the most accurate approach as it takes into account all possible isotope discrimination effects during N uptake. However, its great limitation is the requirement to grow plants only with N from fertilizer. 2. Equation (5.2) takes into account both the isotopic fractionation and soil 15N abundance. It assumes that no isotopic fractionation occurs during fertilizer N uptake or that it does not discriminate between the non-labelled plant and the fertilizer. With respect to (5.1), it has the great advantage that it is not necessary to grow plants only with N from fertilizer. It can be used both in the studies dealing with natural abundance (d15N) and 15N-enriched fertilizer (atom% 15N) (Hauck and Bremner 1976; Shearer and Kohl 1993):
d15 N non—labelled crop—d15 N—labelled crop
d15N non - labelled crop — d15N - labelled fertilizer
When referring to soil, (5.1) and (5.2) are equivalent.
3. Equation (5.3) takes into account both isotopic fractionation and soil 15N abundance. It is similar to (5.2), but the soil 15N is considered to be like atmospheric N2 (Hauck and Bremner 1976; Powlson and Barraclough 1993)
d15N—labelled crop — d15N control crop
d15N - labelled fertilizer
Equation (5.3) is used only in the studies dealing with 15N-enriched fertilizer, while for all other applications (5.2) is recommended.
4. Equation (5.4) does not take into account the isotopic fractionation during soil N uptake, but considers soil 15N abundance. d15N soil is determined before fertilizer addition (Bedard-Haughn et al. 2003; Hauck et al. 1994):
It can be used both in the studies dealing with natural abundance (d15N) and 15N-enriched (atom% 15N) fertilizer. When referring to soil, (5.1), (5.2) and (5.4) are equivalent.
5. Equation (5.5) does not take into account either isotopic fractionation or a possible soil isotopic enrichment/depletion of soil N (Powlson and Barraclough 1993; Wivstad 1999). It is the simplest equation. It can be applied only in the studies dealing with N-enriched fertilizer, in which the fertilizer 15N abundance is very different from atmospheric abundance, in order to cover any isotope discrimination effect or soil N variability. In fact, isotopic fractionation can have a very small influence with 1-10 atom% labelled materials (Barraclough 1995).
atom% 15N excess labelled crop %Nfromfertilizer = atom% 15n excese labelled fertilizer X ^ (5.5)
Section 5.6 provides an exemplification (part of the MESCOSAGR project) of the use of an 15N tracer technique to quantify the relative contribution to plant uptake of SOM and mineralization of added organic fertilizer.
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