Pergamon Appl. Radiat. lsot. Vol. 46, No. 6/7, pp. 611-612, 1995

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THE USE OF 15N AND 32P ISOTOPES IN INVESTIGATIONS OF SOIL FERTILITY UNDER DIFFERENT CROPPING SYSTEMS

ISMAILI MOHAMMED; IBUBUEN JAMAL AND EL ABBADi KARIMA

Department of Biology, Moulay Ismail University, B.P. 4010 Benimhammed, Mekines, Morocco.

The use of isotopes in studies of soil fertility in different cropping systems is important and necessary especially in biological nitrogen fixation, for water stress and in phosphorus experiments. The stable isotope 15N was used to trace the contribution of soil, fertilizer and atmospheric nitrogen to the plant nitrogen content. Quantifying N2 fixation by isotope dilution requires reference to an appropriate non fixing plant similar to the legume in rooting pattern and timing of soil nitrogen assimilation. The 15N and 32p methodologies were used in experiments on legume/wheat rotation and in mixtures to establish the N and P balance and to investigate N fixation, N transfer and competition for soil N between legumes and grasses. The objective of the study was to increase yield and assess the beneficial effect of the mixture and rotation on soil fertility.

Different experiments were conducted to investigate aspects and problems in the use of the 15N and 32p methodology in crop systems. The soil was labelled with different sources of 15N; organic matter enriched with 15N, and by the addition of small quantities of ammonium sulfate enriched with 15N to the soil. The procedure of adding 15N to the soil was different in each experiment. Different cropping systems and treatments were used: medic-wheat mixtures and medic-wheat rotations, at different phosphorus and water stress conditions. N transfer is detected by comparing the percentage of 15N in grass in the mixture with that of grass in monoculture. (Medic refers to medicago truncafa, a clover like legume.)

Dilution of the 15N concentration in grasses grown with legumes compared to mono-grass cultivation suggests N transfer from the legume to the comparison nonlegume. N2 fixation by medic measured by isotope dilution was not affected by P. The isolated medic crop fixed 60% of its N. However, depending on water regime, the legume fixed 73% of its N from the atmosphere in the control treatment and 20 % at a severe water stress treatment. Mixing medic with wheat increased the percentage of nitrogen derived from air in both P and water stress treatments. The percentage of nitrogen derived from atmospheric N2 increased in legume-wheat mixtures with increasing phosphorus application, indicating greater reliance on fixed N2 in the mixture with increasing P rate. The amount of N derived from soil in wheat increased mainly by biological N2 fixation and wheat was very competitive with medica for uptake of soil N. In the mixture, P treatments favoured the growth of wheat more than medic due to the mixed croppings. The medic monoculture was not competing for soil N, however, increasing competition for soil N occurred between both species. Nitrogen transfer varied from 1 to 45% depending on P availability, water

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treatment and number of crops. The medic improved soil N uptake of the following wheat rotation. The 15N atom excess was higher in soil after continuous wheat than that in soil after continuous legume cropping in all P treatments, N input from fixation to the soil amounted to an average of 50 % of the total soil N. Fixed N incorporated into the soil includes N from unrecovered and senescent nodules, root exudates, and fine roots. Moreover, the 15N enrichment of the wheat was lower in all these treatments where medic was grown previously indicating that some of the N absorbed came from N2 fixed by the previous legume.

Table 1. Uptake of mineral N from soil by medic and wheat during the first crop. The total nitrogen derived from the soil in mg/plant and %Ndts refer to total N and %N of medic and wheat plant shoots derived from soil. (PO = 0; P1 = 50; P2 = 100; P3 = 150 kg P205/ha).

Medic Wheat

Pure Mixed Pure Mixed

P TNdfS %Ndfs TNdfs %Ndfs I-~

PO 16.2 40.00 ~.b 19 14.5 20.46 PI 21.2 33.90 12 19 18.0 27.00 P2 39.3 42.68 17.5 18 22.5 34.00 P3 46.0 33.46 15.7 15 25.5 42.00

LSD 3 . 3 NS 0.i NS 1.6 0 . 3 0 ( 0 . 0 5 )

Table 2. Proportion (%Ndfa) and amount (Ndfa; mg/plant) of N2 fixed by the mixed and pure medic; measured by the isotope dilution (I.D.M)

Pure medic Mixed medic

I .D.M. I .D.M. Ndfa %Ndfa Ndfa %Ndfa

TO 3 6 . 8 8 7 3 . 0 3 3 5 . 2 4 9 1 . 5 0 T1 1 5 . 6 0 3 9 . 3 5 3 4 . 1 1 8 8 . 8 6 T2 7 . 3 3 1 9 . 6 0 2 8 . 3 0 8 2 . 2 3

LSD ( 0 . 0 5 ) 5 . 0 9 8 . 4 2 3 . 1 7 3 . 0 3

REFERENCES

Ta, T.C., and Fads, M.A. (1990). Availability of N from 15N-labelled alfalfa residues to three succeeding barely crops under field conditions. Soil Biol. Biochem. Vol. 22 No 6, 835-838.

Varvel, G.E., and Peterson, T.A., (1990). Nitrogen fertilizer recovery by corn in monoculture and rotation systems. Agron. J. 02, 935-938.