Title: isofar
Author: Rosa Marchetti
Last saved by: Anja Schneider
Company: FiBL

16^th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008
Archived at http://orgprints.org/11969

Potentially mineralizable nitrogen is soils green manured with biocidal crops
Marchetti, R., Lazzeri, L., Malaguti, L. , Orsi, A. , & Ponzoni, G. 

Council of Agricultural Research (C.R.A.) Agronomical Research Institute, Modena Section, Viale Caduti in Guerra 134, 41100 Modena, Italy. Current address: C.R.A. Pig Husbandry Research Unit, Via Beccastecca, 345, 41018 San Cesario S/P (Modena), Italy. E-mail: rosa.marchetti@entecra.it

C.R.A. Research Center for Industrial Crops. Via di Corticella, 133, 40129 Bologna, Italy

As 2

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Key words: green manure, N mineralization, Brassicaceae, methyl bromide, metam sodium
Abstract
Biofumigant crops used as green manure, in addition to producing a biocidal effect on some soil-borne pathogens and pests, could represent a source of N for crop nutrition. In two laboratory experiments we compared i) the potentially mineralizable N (PMN) of a silty clay soil after incorporation of glucosinolate-containing (GLS+) and non-containing (GLS–) plants, or after incorporation of metam sodium; and ii) the mineralization rate of different types of soils (silty clay, loam and loamy sand) after green manuring with GLS+ crops. After a 3-month incubation, the PMN of the silty clay soil amended with the GLS+ Brassica juncea was significantly higher than the unamended control and the soil amended with Triticum aestivum and Eruca sativa. Metham sodium, while showing a remarkable nitrification inhibition activity, gave rise to amounts of inorganic N (mainly in the ammonium form) of the same level as B. juncea. Mineralization rate was higher in the loamy sand soil than in the loam and in the silty clay soils. Biofumigant crops used as green manure, by increasing N availability in soil, may represent an interesting source of N for the following crops in organic agriculture.
Introduction
Biofumigant crops used as green manure, in addition to having a biocidal effect on some soil-borne pathogens and pests (Brown & Morra, 1997), may represent a source of N for the following crops. Two laboratory experiments were performed with the following objectives: (i) to compare the potentially mineralizable N (PMN) of a silty clay soil green manured with glucosinolate-containing (GLS+) and non-containing (GLS–) crops. Metam sodium, which is a chemical treatment widely used as an alternative to methyl bromide, was also tested for its effect on soil PMN; (ii) to compare the mineralization rate of different kinds of soils after green manuring with GLS+ crops.
Materials and methods
Experiment I. A silty clay soil was green manured with Indian mustard (Brassica juncea L. Czern., GLS+), garden rocket (Eruca sativa Mill., GLS+), or winter wheat (Triticum aestivum L., GLS–) crop species, in comparison with an untreated soil and with a soil fumigated with metam sodium (methyl isothiocyanate). Experiment II. Three soils belonging to contrasting textural classes: silty clay, loam, and loamy sand, were amended with B. juncea and E. sativa, and compared to an untreated control. Experimental details are reported in Tab. 1. The plant material had been harvested from small field plots at full flowering and finely cut in a grinder mill just before incorporation to soil.
Tab. 1: Selected properties of green manure (GM) and soils used in the two experiments.

GM

Exp.

Added fresh GM
(g L^–1 of wet soil)

Kjeldahl N
(g kg^–1 dry matter)

Plant moisture (% wet weight)

B. juncea | I | 71 | 22.0 | 77.5 | | |
B. juncea | II | 71 | 17.9 | 80.2 | | |
E. sativa | I | 36 | 14.8 | 75.1 | | |
E. sativa | II | 36 | 21.2 | 80.2 | | |
T. aestivum | I | 71 | 13.3 | 74.8 | | |

Soils

Exp.

Bulk density
(kg wet soil L^–1)

Kjeldahl N
(g kg^–1 dry soil)

Initial  moisture (% dry weight)

Sand
(g kg^1)

Silt
(g kg^1)

Clay
(g kg^1)

Silty clay | I | 0.79 | 1.58 | 17.0 | 116 | 478 | 406
Silty clay | II | 0.81 | 1.95 | 22.7 | 98 | 494 | 408
Loam | II | 1.04 | 0.80 | 12.3 | 466 | 415 | 119
Loamy sand | II | 1.11 | 0.82 | 4.3 | 821 | 154 | 25
Amendments were incorporated to a known volume of wet soil in pots kept moist at ambient temperature for a week. In both experiments, PMN was then determined according to Drinkwater et al. (1996): 25 g of wet soil were added to 40-mL scintillation vials, the soil moisture was adjusted at 75% plant available water content and the vials were incubated at 30°C for 3 months (91 days), with 3 replicates for each treatment and date (i.e., at time 0 and after 91 d after the start of the incubation). In the first experiment the PMN was expressed as the net cumulative inorganic N released in soil in the 3-month incubation period. As short-term aerobic incubations quantify a portion of the total PMN (Drinkwater et al., 1996), which, in turn, depends on the total soil organic N content (measured as Kjeldahl N), in the second experiment mineralization rate (MR) was estimated from PMN, as the mean daily amount of net inorganic N released per unit of Kjeldahl N measured at the beginning of the incubation period in both amended and control soils (MR, mg N g^–1 N d^–1= (PMN/91) / soil Kjeldahl N). Statistical analysis was performed using the SAS PROC MIXED (SAS Institute, 1996).
Results
Experiment I. After 3-month incubation the PMN value of the silty clay soil amended with B. juncea was significantly higher than the control (Tab. 2; +51 mg N kg^–1 soil), mainly due to the accumulation of nitrate N. The PMNs of the soil amended with E. sativa and T. aestivum were not significantly different from the control, even though the former was higher, and the latter lower than the PMN of the control. In soil added with metam sodium, the PMN was significantly higher than the control soil, due to a remarkable increase of ammonium, but not of nitrate N.
Tab. 2: Potentially mineralizable N (PMN) in a silty clay soil 3 months after the incorporation of different amendments. Inorganic N is the sum of net cumulative nitrate (NO_3–N) and ammonium (NH_4–N) N.

Treatment

PMN, total inorganic N and N forms
(mg N kg^–1 soil dry weight)^1

| Inorganic N | NO_3-N | NH_4-N
B. juncea | 102ab | 122a | 21b
E. sativa | 68bc | 71b | 3b
T. aestivum | 27d | 41c | 14b
Metam sodium | 111a | 5d | 106a
Control | 51cd | 51bc | 0b
LSD _0.01 | 40.8 | 27.6 | 22.8
1 For each column, means followed by the same letters are not significantly different for P<0.01.
Tab. 3. Mineralization rate (MR, mean daily amount of net cumulative inorganic N per unit of organic N in soil) of different soils green manured with Brassicaceae plant material.
Treatment | MR (mg net cumulative inorganic N g^–1 Kjeldahl N day^–1) in soil^1
| Loam | Loamy sand | Silty clay | Means of Treatment
B. juncea | 0.79b | 1.11a | 0.54cd | 0.74A
E. sativa | 0.71bc | 0.87b | 0.46de | 0.63B
Control | 0.25f | 0.33ef | 0.30f | 0.30C
Means of Soil | 0.59B | 0.77A | 0.43C |
^1 Upper-case letters were used for comparisons of the mean effects, lower-case letters for the comparison of the first-order interaction effects. For each source of variation, means followed by the same letters are not significantly different for P<0.01. LSD_0.01 for comparisons between soils or treatments = 0.104 mg inorganic N g^–1 Kjeldahl N day^–1. LSD_0.01 for comparisons between soils and treatments (soil  treatment effect) = 0.181 mg inorganic N g^–1 Kjeldahl N day^–1.
Experiment II. Green manured soils showed significantly higher MRs than the control soils. The highest MR was observed in the loamy sand (Tab. 3). Not significantly different MRs were observed between control soils.
Discussion
In the first experiment the highest PMN was observed in soil green manured with B. juncea, the lowest in soil green manured with T. aestivum. Trinsoutrot et al. (2000) reported a net N mineralization in soil amended with Brassica napus leaves throughout 168-d incubation, in contrast with net N immobilization for maize straw. The short-term reduction of N availability following the incorporation of winter wheat crop residues, as a result of N immobilisation, is well known, and confirmed by our results. Metam sodium, while showing a remarkable nitrification inhibition activity (in agreement with the findings of Welsh, 1996, in laboratory conditions), greatly stimulated the release of ammonium N from indigenous soil organic matter. In the second experiment, nitrogen mineralization in green manured soil was faster in the loamy sand than in the loam and silty clay soils. This is in agreement with the observations of Pare and Gregorich (1999) for sand soils added with crop residues having a low C:N ratio.
Conclusions
Biofumigant crops used as green manure, by increasing N availability in soil, may represent an interesting source of N for the following crops in organic agriculture.
References
Drinkwater L. E., Cambardella C. A., Reeder J. D., Rice C. W. (1996): Potentially mineralizable nitrogen as an indicator of biologically active soil nitrogen. In Doran J. W., Jones A. J. (eds.): Methods for assessing soil quality, SSSA Spec. Publ. 49, Madison, WI (USA), p. 217–229.
Brown P. D., Morra M. J. (1997): Control of soil-borne plant pests using glucosinolate-containing plants. Adv. Agron. 61:167231.
Pare T., Gregorich E. G. (1999): Soil textural effects on mineralization of nitrogen from crop residues and the added nitrogen interaction. Commun. Soil Sci. Plant Anal. 30:145157.
SAS Institute, 1996. SAS/STAT Software: Changes and Enhancements through Release 6.11. SAS Inst., Cary, NC, pp. 1104.
Trinsoutrot I., Recous S., Bentz B., Linères M., Chèneby D., Nicolardot B. (2000). Biochemical quality of crop residues and carbon and nitrogen mineralization kinetics under non limiting nitrogen conditions. Soil Sci. Soc. Am. J., 64:918–926.
Welsh C. E. (1996): Effects of soil fumigation and N fertilization on nitrification. Thesis, Auburn Univ., Auburn, AL.