The determination of nutrient surplus is one of the indicators of potential N losses from the agricultural system to the environment. An experiment was started in 1998 in Central Italy to evaluate the soil surface N balance of an organic and a conventional low input cropping system over a long term crop rotation. Results at the end of a 6-year crop rotation showed an estimated N surplus in organic system 1.3-2 times higher than in conventional system while N content in the top soil was not different in the two systems, so that organic system should have involved a higher N loss from that soil layer

Benincasa, Paolo

Boldrini, Arianna

Guiducci, Marcello

Tei, Francesco

Tosti, Giacomo

English

Organic Eprints

3rd  QLIF Congress, Hohenheim, Germany, March 20-23, 2007 Archived at http://orgprints.org/view/projects/int_conf_qlif2007.html                                                   Apparent N Balance in Organic and Conventional Low Input Cropping Systems A. Boldrini1, P. Benincasa, G. Tosti, F. Tei, M. Guiducci Key words: nitrogen surplus, nitrogen loss, crop rotation Abstract The determination of nutrient surplus is one of the indicators of potential N losses from the agricultural system to the environment. An experiment was started in 1998 in Central Italy to evaluate the soil surface N balance of an organic and a conventional low input cropping system over a long term crop rotation. Results at the end of a 6-year crop rotation showed an estimated N surplus in organic system 1.3-2 times higher than in conventional system while N content in the top soil was not different in the two systems, so that organic system should have involved a higher N loss from that soil layer. Introduction Organic systems are expected to reduce N leaching (Haas et al., 2002), but this is not always confirmed (Kirchmann and Bergström, 2001). Actually, nitrogen release from organic fertilizer and green manuring might not match crop requirement for both N total supply and timing of N release, with consequent inefficient use of supplied N and increase of residual N exposed to leaching (Benincasa et al., 2004; Jenkinson, 2001; Kirchmann and Bergström, 2001). The determination of nutrient surplus (N supply minus N off-take) at a field scale is often used as an indicator of the potential loss of N (Webb et al., 2000; Heathwaite, 1997; Meisinger and Randall, 1991) and can give an indication of the risks that are associated with specific farming practices. This research is aimed to evaluate the apparent N balance of an organic and a conventional low input system over a long term crop rotation. Materials and methods An experiment was started in 1998 at Perugia (Italy, 43°N, 165 m a.s.l.) to compare an organic (ORG) and a conventional low input (CONV) system in two contiguous fields, both clay loam and with same organic matter and total N contents. Both fields were divided in six sectors (A1, A2, B1, B2, C1, C2) to reproduce the steady-state running of a 6-year rotation in a farm and test several food crops contemporaneously. In each cropping systems a randomized block design with three or four replicates (depending on year and crop) was adopted. The same sequence of cash crops over the six years was adopted in both systems (Table 1). Nitrogen supply to the system was assured by green manuring (legumes, pure or mixed with non-legumes) and/or poultry manure in ORG and by green manuring (only until 2000) and mineral fertilizers in CONV. Above ground N accumulation of any crop and its partitioning between commercial biomass and crop residues were determined by plant sampling and analysis of organic N d. m. content (Kjeldhal method).  1Department of Agricultural and Environmental Sciences, University of Perugia, 06121 Perugia, Italy, Internet www.agr.unipg.it, E-mail corresponding author: arianna.boldrini@agr.unipg.it    Archived at http://orgprints.org/9885/   3rd  QLIF Congress, Hohenheim, Germany, March 20-23, 2007 Archived at http://orgprints.org/view/projects/int_conf_qlif2007.html   Tab. 1: Six-year crop rotations in the six field sectors in organic (ORG) and conventional low input (CONV) cropping systems. Green manure crops (GM) were adopted in ORG and CONV until 2000, only in ORG afterwards. Nitrogen supply (kg ha-1) from fertilizers in ORG/CONV system is reported in brackets.  Years  Field sectors 1999 2000 2001 2002 2003 2004 A1  common bean (40/0) spelt       (40/40) GM1+maize (40/150) GM4+soybean (0/0) GM1+pepper (0/200) wheat       (0/80) A2  common bean (40/0) wheat       (80/80) GM1+pepper (40/175) GM4+maize (40/40) GM1+tomato (0/200) wheat       (0/80) B1  field bean (0/0) GM3+pepper (100/100) pea         (40/0) wheat       (80/80) GM2+maize (0/150) GM3+tomato (60/160) B2  field bean  (0/0) GM3+maize (100/100) field bean (40/0) wheat      (80/80) pea         (0/0) GM3+pepper (60/200) C1  GM1+pepper (0/70) common bean (40/0) spelt        (80/80) GM1+tomato (60/200) wheat       (40/80) field bean   (0/0) C2  GM1+millet (20/135) common bean (40/0) wheat       (80/80) GM1+pepper (60/200) wheat       (40/80) GM1+maize (40/150) GM1: field bean; GM2: field bean+rapesed; GM3: hairy vetch; GM4: barley.  In legumes, N derived from atmosphere (Ndfa) via symbiotic N2 fixation was assumed to account for 80% of accumulated N, based on data reported by Vance (1988) and by Cazzato et al (2003) for the Italian environment, and on our own data (not yet published) analysed according to Mϋller and Thorup-Kristensen (2002) on leguminous and grass species grown contemporarily for several years in field experiments carried out in the same location of the present trial. The Ndfa accumulated in legume roots is not considered here. For this reason, due to the different frequency of green manure crops in the two systems (Table 1) the underestimate of N input by legume roots Ndfa is most likely higher in ORG than in CONV. Apparent residual N in the soil (i.e. the soil-crop component of the soil surface budget) (Aarts et al., 2000) was calculated at the end of each crop cycle as: ∆N = A + B - C; where ∆N = apparent residual N in the soil per unit area (kg ha-1); A = N input as mineral and organic fertilizer (kg ha-1); B = estimated legume Ndfa (kg ha-1); C = N off-take with commercial yield removal (kg    ha-1). The soil N content (both organic and mineral N) was determined in each sector at the end of the 6-year crop rotation by sampling four 0-0.40 m soil cores per each sector. Data were submitted to analysis of variance according to a hierarchical design (crops within systems). Results As an average over the six field sectors, the 6-year cumulated total N input (i.e. N from fertilizers + legume Ndfa) (Table 2) was similar in the two systems (594 kg ha-1 in ORG vs 603 kg ha-1 in CONV), with estimated Ndfa component in ORG about 3.5 times higher than in CONV, the N off-take was 17% lower in ORG than in CONV, while the ∆N was 34% higher in ORG than in CONV. In particular, except for sector A1, field sectors in ORG showed a cumulated ∆N 1.3-2 times higher than in CONV. Between-sectors variability was observed for all N balance components, with sectors sorted pretty similarly in both systems, except for Ndfa that in CONV was affected by occasional green manuring in some sectors. 3rd  QLIF Congress, Hohenheim, Germany, March 20-23, 2007 Archived at http://orgprints.org/view/projects/int_conf_qlif2007.html   Table 2: Cumulated 6-year N input from fertilizers, estimated legume N derived from atmosphere, N off-take and ∆N (i.e. total input minus off-take) in six field sectors of organic (ORG) and conventional low input (CONV) cropping systems. N from fertilizers (kg ha-1) Legume Ndfa (kg ha-1) N off-take         (kg ha-1) ∆N             (kg ha-1) Field sectors ORG CONV  ORG  CONV  ORG CONV ORG  CONV A1 120  470  335  66  284  300  172  236 A2 200  575  303  38  450  579  53  34 B1 280  490  368  151  341  486  307  155 B2 280  380  373  237  302  341  351  276 C1 220  430  386  110  345  399  261  141 C2 280  645  421  26  369  406  332  264 mean 230  498  364  105  349  419  246  184 Pooled SD  31 74  58  38  45  48  69  71  Both organic and mineral N contents of the 0-0.40 m top soil at the end of the 6-year rotation were substantially the same in both ORG and CONV (Table 3).  Table 3: Organic, mineral and total N contents in the top soil layer (0-0.40 m) in organic (ORG) and conventional low input (CONV) cropping systems at the end of the 6-year crop rotation. Systems  Organic N Kg ha-1 Mineral N Kg ha-1 Total N Kg ha-1 ORG 4065  29  4095 CONV 4188  34  4222 Pooled SD   561 8  563 Discussion The higher 6-year cumulated ∆N in ORG with respect to CONV was determined by both the lower cash crop N off-take and the imprecise N availability from green manuring in ORG. The exception of sector A1, where ∆N was higher in CONV than in ORG, is mainly due to vascular diseases and a heavy nematode attack on sweet pepper in 2003, so that a great part of commercial yield was damaged and therefore left in the field and then incorporated into the soil. The lower cash crop N off-take in ORG resulted from a generally lower commercial yield of many crops as a consequence of lower initial growth (especially in summer crops, due to low early N availability and/or to scrubby organic transplants) and/or of higher weed competition (especially in grain legumes). The imprecise N availability from green manuring concerned both the total N supplied from green manure crops and the timing of N release from incorporated biomass which both showed a high inter-annual variability (Benincasa et al., 2004). Differently from CONV, where mineral fertilizer at low rates generally allowed high N uptake efficiency, in ORG sometimes N supply by green manuring exceeded cash crop total requirements and sometimes N release from incorporated biomass did not cover cash crop early N requirements, with consequent reduction of cash crop N uptake efficiency. Since at the end of the 6-year rotation both organic and mineral N contents in the 0-0.40 m top soil layer were similar in the two systems, it can be argued that the higher ∆N recorded in ORG should have caused a higher loss of N from that soil layer. 3rd  QLIF Congress, Hohenheim, Germany, March 20-23, 2007 Archived at http://orgprints.org/view/projects/int_conf_qlif2007.html   Conclusions In our experiment, organic system involved a higher N surplus (i.e. N input by fertilizers plus estimated legume Ndfa minus N off-take) with respect to the conventional low input system. However, the higher N surplus did not involve an important difference between systems in the top soil N content (both organic and mineral) at the end of the 6-year rotation. For this reason organic system in this experiment was estimated to involve a higher N loss from the top soil.  Acknowledgments Research founded by the FISR SIMBIO – VEG Project (2005-2008). References Aarts H. F. M., Habekotté B., Van Keulen H. (2000): Nitrogen management in the “De Marke” dairy farming system. Nutrient Cycling in Agroecosystems 56: 231-240. Benincasa P., Boldrini A., Tei F., Guiducci M. (2004): N release from several green manure crops. Proceedings of the VIII ESA Congress - Addendum, Copenhagen, Denmark, 11-15 July 2004, 971-972. Cazzato, E., Venticelli, P., Corleto, A., 2003. N2 fixation of annual fodder legumes in Mediterranean environment. Note 2. Comparison between difference and isotope dilution methods and evaluation of two grass species as reference crops. Rivista di Agronomia 37, 63-68. Haas G., Berg M., Köpke U. (2002): Nitrate leaching: comparing conventional, integrated and organic agricultural production systems. Agricultural effects on ground and surface waters: research at the edge of science and society. Proceedings of an international symposium, Wageningen, Netherlands, October 2000, 2002, 273:131-136. Heathwaite A. L. (1997): Sources and Pathways of Phosphorus Loss from Agriculture. In: Tunney H., Carton, Brookes P. C., Jonhston A. E. (eds): Posphorus Loss from Soil to Water, 465 pp. Jenkinson D. S. (2001): The impact of humans on the nitrogen cycle, with focus on temperate arable agriculture. Plant and Soil 228: 3-15. Kirchmann H., Bergström L. (2001): Do organic farming practices reduce nitrate leaching? Comunications in Soil Science and Plant Analysis 32-7/8: 997-1028. Meisinger J. J., Randall G. W. (1991): Estimating Nitrogen budgets for soil-crop systems. In: Managing Nitrogen for Groundwater Quality and Farm Profitability. Müller and Thorup-Kristensen (2002): Total N difference method and N isotope dilution method – A comparative study on N-fixation. Mitteign. Dtsch. Bodenkundl. Gesellsch., 98, 23-24. Vance C.P. (1988). Legume symbiontic nitrogen fixation: agronomic aspects. In Spain H. P., Kondorosi A., Hooykaas P. J. J. (eds): The Rhizobiaceae. Kluwer Academic. Publishers, Dortrecht, 509-530. Webb J., Harrison R., Ellis S. 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Apparent N Balance in Organic and Conventional Low Input Cropping Systems

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