Scientific registration n : 1728 Symposium n : 07 Presentation : poster

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1 Scientific registration n : 1728 Symposium n : 07 Presentation : poster Long-term crop rotation effects on oc, n and p in soil granulometric fractions Effets à long terme des rotations culturales sur le carbone organique, l azote et le phosphore dans les fractions granulomètriques du sol ROSELL Ramón A. 1,3, GALANTINI Juan A. 2,3, SUÑER Liliana G. 2,3, PISANDELLI Pablo 3 1 CONICET, 2 CIC, 3 LAHBIS Dpto. de Agronomía-UNS, 8000 Bahia Blanca, Argentina INTRODUCTION Soil organic matter consist of a number of fractions, which can be separated by granulometric wet sieving and which have different properties among them. The quantities and quality of the various types of organic matter have an important effect on biological productivity and soil health (Doran and Parkin, 1994). For over a century, SOM has been chemically characterized, with some interest being shown in pedogenesis research but with scarce significant agronomic application. Granulometric soil fractions can help to characterize the main organic matter pools, separating active, fresh or particulate organic matter (POM) and non-active (humified) fractions. Numerous data demonstrate that the larger portion of SOM is found in the fine fraction having a low turnover rate. Less transformed organic materials are more dynamic and apparently have an active roll in soil fertility; it provides much of the energy for soil organisms and available nutrients during the crop cycle. Crop sequences may have an important impact on SOM dynamics, nutrient cycling and an efficient use of water. The objective of this research was aimed at understanding long-term dynamics of soil OC, N and P under different agronomic systems. MATERIALS AND METHODS Experimental site The experimental site is located in and is representative of the central-southern semiarid Pampean region (33-41 S; W) of Argentina. The climate is temperate, continental semiarid. Mean annual temperature and rainfall are 15.2 C and 650 mm ( ), respectively, with higher precipitation in fall and spring seasons as despited in Figure 1. Predominant soils belong to a complex of Entic Haplustolls, coarse loamy family, mixed, mesic, with a horizon sequence A-AC-C-Cca and a caliche layer at an average of 0.7 m deep. 1

2 Crop sequences The soils had been seeded to consociated pastures (Medicago sativa L., Phalaris sp L. and Festuca arundinacea Schreb.) during several decades before the experiments were initiated. Later on, native grasses (Stipa spp, Bromus spp, and Medicago polimorfa L.) were predominant in the soil. Starting in 1983, the following treatments were established: - Ref, an uncultivated check or reference plot near each rotation treatment to minimize any possible soil texture difference, which did not receive neither tillage nor herbicide applications. -Pa, continuous mixed pasture (Medicago sativa L., Phalaris sp L. and Festuca arundinacea Schreb.) from 1983 up to date. Pa-C, 5.5 years of mixed pasture (like in Pa) and 6.5 years of annual crops: sorghum (Sorghum bicolor (L.) Moench) - wheat (Triticum aestivum)- wheat - vetch (Vicia sativa L.) + oat (Avena sativa L.) - wheat. - W-C, wheat-different crops [native grasses or oat or soybean (Glicine max) or sorghum or oat-vetch] one year each. No fertilizer were applied to the treatments. For annual crops, the following tillage practices were applied: harrowing (off-set) after harvesting; chisel and disk plows for seed bed preparation; deep furrow seed drilling and chemical and mechanical (for summer crops) weed control. Crop residues were not removed. Three blocks were located in each treatment and three composite samples were obtained in each one. Soil samples ( m deep) were obtained in November of 1991, 1992 and 1994; air-dried and sieved (2 mm). Soil granulometric fractions Soil granulometric fractions by wet sieving were obtained. Fifty-gram soil samples were treated with 100 ml distill water. Ten glass beads to facility soil dispersion were added and the mixture was shaken one hour. The suspension was sieved (100 µm) and rinsed with water. Two soil granulometric fractions were obtained: fine fraction (FF, <100 µm) and coarse fraction (CF, µm). Both of them were dried at 65 C. Chemical determinations The following chemical determinations were made: - SOC, total organic carbon by dry combustion (LECO Carbon analyzer, LECO Corporation St. Joseph, MI, USA); HOC, humified organic carbon present in the FF (LECO Carbon analyzer); POC, particulate organic carbon present in the CF (LECO Carbon analyzer); Nt, total nitrogen (Bremner and Mulvaney, 1982); P e, extractable phosphorus (Bray and Kurtz, 1945); P o and P i, organic and inorganic P (Saunders and Willians, 1955); P te, total P from perchloric acid treatment (Olsen and Sommers, 1982) and Induction Coupled Plasma (ICP) determination; P oc, occluded P by difference between P t and P o +P i ; and ph. The statistical analysis of the results was carried out by applying the two-way (block treatment) ANOVA (BMDP, 1992). The pair means were compared with the Tukey test. RESULTS AND DISCUSSION Organic carbon changes Table 1 presents the change of OC and total N in the fine and coarse fractions of an Entic 2

3 Haplustoll under several crop sequences after 9 years. The HOC (fine fraction) was the largest and less variable fraction of the SOC; it had higher values under pasture (Pa) than under tilled treatments. The OC was maintained in these treatments, except in 1994 for W-C. In this case, oat-vetch were seeded producing a slight non-significative increase of OC in the soil. The Pa treatment showed also a slight non-significative increase of OC, compared with Ref. The level of HOC in the Pa treatments was maintained throughout the experiment, even increasing in On the other hand, POC decreased significatively in 1991 and The changing climatic conditions produced annual variability in the OC levels. Figure 1 shows that the 1991/92 period had higher precipitation and, consequently, more dry matter production and decomposition. Soil total nitrogen (Nt) decreased under cropping but did not change under Pa. This treatment showed a relative Nt increase in the POM fraction. Total N decreased sharply (more than OC) in the soil fine fraction. The C:N ratio in the fine fraction increased with cultivation in all treatments. On the other hand, C:N decreased in the coarse fraction of all treatments. That means that the SOC was used as energy source and lost by oxidation in the coarse fraction, meanwhile N was retained in the microbial tissue. Dynamics of the P forms Table 2 indicates the long-term dynamics of P forms in the soil and the granulometric fractions of an Entic Haplustoll under several crop sequences after 9 years. The Ref plots showed significant high levels of different P forms and ph in comparison with all treatments. Since 1983, when the experiment began, there were P-forms concentration changes. Inorganic P was similar in the FF of all plots in Later on, in 1994, decreased significatively; Po increased simultaneously. In the coarse fraction both Po and Pi decreased in the rotations with respect to the Ref plot. This behavior was due to the exportation of the harvested crops (grains and pasture bails). Also, Po decomposition in the coarse fraction may have favored the incorporation of P compounds in the FF. At the end of Pa-C agricultural cycle the values of Pi in the FF were lower than those from the Ref soils. Pi increased gradually in the rainy period of Pi and Po in W-C had similar values as in the Ref plots in the coarse fraction. On the contrary both P forms decreased in the FF. It is possible that tillage favors the liberation of occluded P which is normally found in the FF (Galantini and Rosell, 1997). Furthermore, these changes could have been activated by a ph decrease. As expected, extractable P (Pe) was higher in the Ref plots than in any cultivated one. A correlation was observed between Pa treatment and Po in the coarse fraction, phenomenon already observed in other soils of the region. Conclusion The SOM in the fine fraction was the most recalcitrant. On the other hand, POM participated in SOM changes related to crop rotation, plant residue input and water availability. Phosphorus in coarse fraction dropped sharply in all cases, mainly the organic one. The fine fraction Po was unaffected by tillage treatment but increased under pasture treatment. 3

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5 References BMDP Statistical Software Inc., BMDP Statistical Software Manual. Los Angeles, CA, USA. Bray, R.H. and Kurtz, L.T., Determination of total, organic and available forms of phosphorous in soils. Soil Sci., 59: Bremner, J.M. and Mulvaney, C.S., Nitrogen total. In: Page et al. (Editors) Methods of Soils Analysis. Part 2. Agronomy 9. 2 nd edition. Madison. Wisconsin. USA. pp Doran J.W. and F. Parkin Defining and assessing soil quality. In Defining Soil Quality for Sustainable Environment (Eds. J.W. Doran, D.C. Coleman, D.F. Bezdicek and B.A. Stewart) SSA Special Publication Number 35, SSSA-ASA, Madison, WI, USA. Pp Galantini J.A. and Rosell R.A Organic fractions, N, P, and S changes in a semiarid Haplustoll of Argentine under different crop sequences. Soil and Tillage Research 42: Olsen, S.R. and Sommers, L.E., Phosphorus. In: Methods of soil analysis. Page et al. (Editors), Part 2. Agronomy 9, 2 nd edition, Madison, Wisconsin, USA. pp Saunders, W.M.H. and Williams, E.G., Observations on the determination of total organic phosphorus in soil. J. Soil Sci., 6: Table 1: Change of organic carbon (OC) and total nitrogen (N) in fine and coarse fractions of an Entic Haplustoll after 9 years of cropping. Fine fraction Coarse fraction OC N C:N OC N C:N POC:SO C g kg -1 g kg -1 Ref Pa Pa-C W-C LSD

6 Table 2: Long-term dynamics of P forms in soil and granulometric fractions of an Entic Haplustoll after 9 years of cropping. Fine fraction Coarse fraction Whole soil Po Pi Po Pi Pe ph µg P g -1 Ref Pa Pa-C W-C LSD RAINFALL, mm YEAR Figure 1: Annual rainfall in the Semiarid Pampean Region, Bordenave experimental site Key-words: soil organic carbon, granulometric fractions, crop rotations Mots clés : matière organique du sol, granulomètrie, rotations culturelles 6