Traditionally, the assessment of soil sustainability and the potential impact of cultivation are based upon the application of chemical procedures. In the absence of a biological context, these measurements offer little in understanding longterm changes in soil husbandry. Detailed microcosm investigations were applied as a predictive tool for management change. The microcosms were designed with homogenised soils treated with organic amendments. Key soil functional relationships were quantified using stable isotope techniques, biochemical measurements and traditional approaches

Killham, K

May-Roberts, S

Paton, G

Robertson, J

English

Organic Eprints

Aspects of Applied Biology 79, 2006What will organic farming deliver? COR 2006307Soil sustainability in organic agricultural productionS MAY-ROBERTS1, J ROBERTSON1,2, K KILLHAM1 & G PATON11Department of Plant & Soil Science, School of Biological Sciences, University of Aberdeen, Aberdeen AB24 3UU, UK2Kintail Land Research FoundationSummaryTraditionally, the assessment of soil sustainability and the potential impact of cultivation are based upon the application of chemical procedures. In the absence of a biological context, these measurements offer little in understanding long-term changes in soil husbandry. Detailed microcosm investigations were applied as a predictive tool for management change. The microcosms were designed with homogenised soils treated with organic amendments. Key soil functional relationships were quantiﬁ  ed using stable isotope techniques, biochemical measurements and traditional approaches.  Keywords: Organic management, conventional management, microbial activity microbial biomass, ecological indicators, soil healthIntroduction An ongoing challenge for both scientists and policy makers is to preserve agricultural efﬁ  ciency whilst using the natural resilience of the soils to establish sustainable production in unimproved and degraded systems, in the face of continuous population growth (Grifﬁ  ths et al., 2001). From this, a particular interest has grown in understanding soil processes particularly in terms of the relationship between biodiversity and function. It is now considered to be the key in estimating the inﬂ  uence of farming practices on the fertility and quality of soil, and thus on the environment (Bohme et al., 2005).Productivity of soils, with regard to their utilization is determined by properties and attributes of the whole soil body (Arnold et al., 1990). Thus, a chief concern for societies should be the impact of soil management practices on the physical, chemical and biological processes of soils that inﬂ  uence the sustainability of agriculture (Karlen et al, 2003).Materials and MethodsComparisons of conventional and organic based production systems were made using two neighboring farms, Kirkbog and Kirkland, located in Dumfries and Galloway. The selected ﬁ  elds for soil analyses were adjacent to each other to ensure the same pedological conditions as farm management varied. The crop rotation of the conventional system was similar to the organic system.  The agricultural management of the four ﬁ  elds differed but was typical of organic and 308conventional practices in the UK. In 2005, the rotational ﬁ  eld from the organic farm had been utilized to grow grass, peas and triticale in rotation for the past three years. The rotational ﬁ  elds of the conventional farm had been utilized to grow grass and maize in rotation. At the time of sampling the rotational ﬁ  elds of both the organic and conventional farm were under grass ley. Pasture ﬁ  elds of the conventional farm had not been ploughed for 16 years, whereas pasture ﬁ  elds of the organic system had not been ploughed for 25 years, both were utilised as permanent pasture for livestock.  A microcosm study was established to determine differences in key functional soil processes between conventional and organic husbandry. Soils were amended with a 15N plant residue (rye grass) and incubated for a 90-day period with growing barley plants acting as a plant sink for N.   Microcosms were destructively sampled, to test a large suite of indicators, as a variety of parameters should be investigated when considering the impact of management on soil sustainability. Both available nitrate (NO3--N) and ammonium (NH4+-N) were determined by colorimetric methods after extraction from soil with 1 M KCl. Microbial biomass C (MBC) was determined by chloroform fumigation extraction method, using 0.5 M K2SO4 as an extractant. The C contents in the fumigated and non-fumigated were estimated as reported by Vance et al. (1987).Acid phophatase (PNP), alkaline phophatase (PNP*) and dehydrogenase (DH) activities were measured as indicators of soil enzymatic activity. Acid and alkaline phosphatase was assayed according to methods described by Skujins. et al. (1962). The procedure involves the spectrophotometric determination (wavelength 410 nm) of p-nitrophenol (pNP) released by 1 g of soil during 60 min at 37°C. Dehydrogenase activity was determined using methods adapted from Casida et al. (1964) and Trasar-Cepeda et al. (2000), and based on the spectrophotometric determination of iodonitrotetrazolium formazan (INTF) released by 1 g of soil during 24 h at room temperature.Results and DiscussionTable 1. Biological and chemical indicators of organically and conventionally managed pasture soils during a 90-day incubation period.Day 0 Day 45 Day 60Organic Conventional Organic Conventional Organic ConventionalBiologicalindicatorsBiologicalPNP (µmol pNPg-1soil h-1)0.00 ± 0.00 0.00 ± 0.00 0.04 ± 0.00 0.03 ± 0.00 0.11 ± 0.01 0.11 ± 0.01PNP*(µmol pNPg-1soil h-1)0.09 ± 0.00 0.09 ± 0.00 0.09 ± 0.00 0.08 ± 0.00 0.25 ± 0.02 0.22 ± 0.05DH (µmol INTF g-1soil h-1)0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 0.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00MBC (mg 100-1) 115 ± 4.13 135.15 ± 4.14 304.99 ± 7.12 168.39 ± 8.77 297.14 ± 35.3 176.64 ± 10.27Available nutrientsNO3--N (mg kg-1) 27.22 ± 0.26 21.84 ± 0.082 1.30 ± 0.07 0.4 ± 0.00 3.07 ± 0.22 2.09 ± 0.24NH4+-N (mg kg-1) 4.12 ± 0.41 5.43 ±0.16 1.73 ± 0.49 1.98 ± 0.22 2.5 ± 0.33 2.18 ± 0.01309Table 2.  Biological and chemical indicators of organically and conventionally managed rotational soils during a 90-day incubation periodDay 0 Day 45 Day 60Organic Conventional Organic Conventional Organic ConventionalBiological indicatorsPNP (µmol pNPg-1soil h-1)0.00 ± 0.00 0.00 ± 0.00 0.03 ± 0.00 0.03 ± 0.00 0.15 ± 0.04 0.03 ± 0.00PNP*(µmol pNPg-1soil h-1)0.08 ± 0.00 0.07 ± 0.00 0.08 ± 0.00 0.08 ± 0.00 0.29 ± 0.01  0.22 ± 0.05 DH(µmol INTF g-1soil h-1)0.01 ± 0.00 0.02 ± 0.00 0.01 ± 0.00 0.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00MBC (mg100-1) 151.09 ± 2.93 140.81 ± 3.03 369.23 ± 9.94 229.19 ± 1.56 292.07 ± 35.3 248.11 ± 1.82Available nutrientsNO3--N (mg kg-1) 20.98 ± 0.57 19.5 ± 0.77 0.47 ± 0.03 0.29 ± 0.07 2.92 ± 0.14 2.74 ± 0.74 NH4+-N (mg kg-1) 2.22 ± 0.05 3.27 ± 0.12 2.04 ± 0.27 2.88 ± 0.05 2.3 ± 0.2 2.09 ± 0.08Despite the fact that soil management inﬂ  uences most soil parameters, several authors (Nannipieri, 1984; Bergstrom et al., 1998) reported that microbial biomass and enzyme activity respond more quickly to management and land use. Thus, microbial biomass and the enzyme activities are considered to be more sensitive indicators of soil sustainability than chemical properties (Beyer et al., 1999).  In this study, enzymatic activities (Tables 1 and 2) as indicators of microbial activity presented broadly similar patterns for pasture and rotational ﬁ  elds, irrespective of management. So far, weak correlations were found between dehydrogenase and microbial biomass C, which may be due to a signiﬁ  cant proportion of dormant microbes. Preliminary investigations of available soil nitrogen in the forms of nitrate and ammonium are presented in Tables 1 & 2. The effect of management was more pronounced for nitrate; furthermore, concentrations of ammonium decreased during the ﬁ  rst 45 days, suggesting that a high level of nitriﬁ  cation in an aerobic environment is being maintained in both soil types.Soil nitrate availability was always higher in organic than in conventional ﬁ  elds in both rotation and conventional, although this was insigniﬁ  cant. A possible explanation of the difference in nitrate concentration may be to the higher activity of microbial nitrifying bacteria, which may be affected by different fertiliser application in the conventional system (Chao et al., 1996), although residual N effects from previous management regimes should also be considered. For matched soils under contrasting management practices, there have been no marked changes, to date, in terms of soil mineral N concentrations, microbial biomass C and enzyme (dehydrogenase) activities. Statistical analysis revealed differences only with time, but not with management practices. This suggests that a period of 5 years of contrasting management was not enough to produce consistent differences in soil microbial activity.This may well relate to the intrinsic dynamics of change due to the organic conversion rather than a lack of change.  However, by continuing to monitor biomass carbon, the study is focusing on the main barometer of change in the soil.  In terms of N mineralisation, there is no evidence to date to suggest this is enhanced under organic management, although 15N data will conﬁ  rm whether the 310efﬁ  ciency of N transfer from the residue to the crop is changed under organic husbandry.AcknowledgementsThe authors wish to thank John G Skea and Sons, and Stewart Jamieson for their kind collaboration with this study. The Kintail Land Research Foundation is gratefully acknowledged for providing ﬁ  nancial support to the organic research programme at the University of AberdeenReferencesArnold R W. 1990. Processes that affect soil morphology. Soils on a warmer Earth. Proceedings of an international workshop, Nairobi, pp. 31–38.Beyer L, Sieling K, Pingpank K. 1999. The impact of a low humus input level in arable soils on microbial properties, soil organic matter quality and crop yield. Biology and Fertility of Soil. 28: 156–161.Bergstrom D W, Monreal C M, King D J. 1998. Sensitivity of soil enzyme activities to conservation practices. Soil Science Society of America Journal 62:1286–1295.Bohme L, Langer U, Bohme F. 2005. Microbial biomass, enzyme activities and microbial community structure in two European long-term experiments. Agriculture Ecosystems & Environment 109:141–152.Casida L E, Klein D A, Santero T. 1964. Soil dehydrogenase activity. Soil Science 98:371–376.ChaoW L, Tu H J, Chao C C, 1996. Nitrogen transformations in tropical soils under conventional and sustainable farming systems.  Biology and Fertility of Soils 21:252–256.Grifﬁ  ths B S, Bonkowski M, Roy J, Ritz K.  2001.  Functional stability, substrate utilisation and biological indicators of soils following environmental impacts. Applied Soil Ecology 16:49–61. Karlen D L,  Ditzler C A, Andrews S S. 2003. Soil quality; why and how? Geoderma 114: 145–156.Nannipieri P. 1984. Microbial biomass and activity measurement in soils: ecological signiﬁ  cance. In Current Perspectives in Microbial Ecology, pp. 512–521. Eds M J Klug and C A Reddy.  American Society of Microbiology, Washington.Skujins J J, Braal L, McLaren A D.  1962. Characterization of phosphate in a terrestrial soil sterilised with an electron beam. Enzymologia 25:125–133.Trasar-Cepeda C, Leirós M C, Seoane S, Gil-Sotres F. 2000. Limitations of soil enzymes as indicators of soil pollutions. Soil Biology and Biochemistry 32:1867–1875.Vance E D, Brookes P C, Jenkinson D S. 1987. An extraction method for measuring soil microbial biomass C. Soil Biology and Biochemistry 19:703–707.

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