Ó Springer 2005
Nutrient Cycling in Agroecosystems (2005) 73:181–189 DOI 10.1007/s10705-005-0593-z
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Effect of green manure addition on soil organic phosphorus mineralisation Parmjit S. Randhawa1, Leo M. Condron1,*, Hong J. Di1, Sokrat Sinaj2 and Roger D. McLenaghen1 1
Agriculture and Life Sciences, P.O. Box 84, Lincoln University, Canterbury 8150, New Zealand; 2Institute of Plant Sciences, Swiss Federal Institute of Technology Zurich (ETHZ), CH-8315, Eschikon-Lindau, Switzerland; *Author for correspondence (e-mail:
[email protected]; fax: +00-64-3-325-3607)
Received 6 October 2004; accepted in revised form 24 June 2005
Key words: Flux approach, Green manure, Isotopic dilution, Lupinus angustifolius, Organic P mineralisation, Zea mays
Abstract The mineralization of organic forms of phosphorus (P) in soil contributes significantly to plant P uptake, although quantification of organic P mineralisation has been impeded by methodological difficulties. An isotopic dilution method, based on tracer kinetic theory, was used to study the organic P flux rates for unamended and green manure amended soil in an incubation experiment carried out over 35 days. Firstly, the specific activity (SA) between two points of time as affected by dominant biological processes (under conditions of constant soil respiration rates) was determined in a series of successive labelling incubation experiments. Secondly, the instantaneous specific activity (instSA) between two points of time was also determined by shaking an unlabelled soil with 33P, i.e. soil samples that were not labelled with 33P, but kept under conditions identical to the labelled soil samples. This gives the net size of the exchangeable inorganic P pool in the labelling experiments at two points of time during incubation. The SA dilution in the labelling experiments at a constant exchangeable inorganic P pool between two times was attributed to the release of non-labelled inorganic P from the organic P pool by mineralisation. The daily gross organic P mineralisation rates for the 21 days of incubation were 0.06 and 0.27 mg P kg1 day1 for unamended and green manure amended soils, respectively. The study demonstrated that green manure amendment contributed to an overall increase in soil P availability through enhanced organic P mineralisation. Abbreviations: P – phosphorus; CP – water-soluble inorganic P; SA – specific activity; instSA – instantaneous specific activity; Fin – influx; Fout – out-flux; FR – flux rate; Q – exchangeable inorganic P pool; t – time; R – total 33P introduced in the unlabeled soil
Introduction The release of inorganic phosphorus (P) from the mineralisation of organic P is an important factor that influences the availability of phosphorus in agroecosystems (Di et al. 2000; Frossard et al. 2000). However, the measurement of organic P
mineralisation by chemical methods is severely hampered by the high reactivity of phosphate ions, which are rapidly adsorbed on soil surfaces after their release to soil solution (Parfitt et al. 1994; Frossard and Sinaj 1997). In the past two decades, several studies have been carried out to investigate the potential use of isotopic dilution techniques to
182 quantify organic P mineralisation (Walbridge and Vitousek 1987; Lo´pez-Herna´ndez and Nino1993; Frossard et al. 1996; Lo´pez-Herna´ndez et al. 1998; Oehl et al. 2001b). Isotopic dilution also has limitations due to ongoing P transformations that occur during the incubation process (Di et al. 1997; Frossard et al. 2000). Oehl et al. (2001b) discussed the use of isotopic exchange kinetics (IEK) batch experiments (physico-chemical processes) and the labelled incubation experiment (physicochemical and biological processes) to estimate gross soil organic P mineralisation. This study also reviewed the previous studies on organic P mineralisation and suggested modifications to assign the measurements specifically to mineralisation. Di et al. (2000) proposed an isotopic dilution technique based on tracer kinetics to measure readily exchangeable soil inorganic P pool which under certain experimental conditions can estimate organic P mineralisation. Green manuring is an agronomic practice whereby a crop is grown and incorporated into the soil at an early stage of development. In temperate conventional and organic cropping systems, it has been demonstrated that green manures can be used to improve the retention and supply of nitrogen (N) (Thorup-Kristensen et al. 2003; Fowler et al. 2004). In organically managed systems soil organic P mineralisation has been shown to make a significant contribution to plant P requirements (Oehl et al. 2001a), while the inclusion of green manure crops enhances biological P cycling in soil and improves the dissolution and bioavailability of sparingly-soluble phosphate rock (Cavigelli and Thien 2003; Kamh et al. 1999). As part of a wider investigation of green manuring and P availability in organic cropping systems, the specific objective of this study was to use an isotopic dilution technique to quantitatively determine the impact of green manure amendment on the rate of soil organic P mineralization. Materials and methods Theory The concept of soil N mineralisation suggested by Mary and Recous (1994), as adopted and explained for organic P mineralisation by Oehl et al. (2001b), was used in the present study. Basal
organic P mineralisation can be defined as gross mineralisation in the absence of flush effects, that is, at a constant soil respiration rate. It represents the basal potential of a soil to deliver inorganic P from organic P to the soil solution. Re-mineralisation takes place due to recycling of microbial P during microbial death and predation and signifies mineralisation of recently synthesized organic P (Mary and Recous 1994). Biological P immobilisation corresponds with the assimilation of P by soil micro-organisms. The estimation of gross soil organic P mineralisation was attempted by using a single compartment model described by Di et al. (2000). This flux approach is based on the principle of tracer kinetics in which the changes in the inorganic P pool are measured on the basis of changes in specific activity (SA). Specific activity is expressed as the ratio of tracer (33P) and tracee (31P) (Bq lg1P) in the labelled inorganic P pool. In this approach for labelled soil, the SA dilution is attributed to basal organic P mineralisation (in the absence of any other dominant flux) if the size of the readily exchangeable inorganic P pool calculated from the instantaneous SA (instSA) is not significantly different over a period of time. The gross organic P mineralisation quantification is based on the following assumptions: (i) Soil organic P is not labelled by isotopic exchange. The release of inorganic P to soil solution due to organic P mineralisation decreases the specific activity of inorganic P in soil solution; (ii) Phosphorus isotopes (33P and 31P) behave identically in the soil (i.e. microbes do not discriminate between the isotopes). When a quantity R of carrier-free 33P ion is added in to a soil, 33P will exchange with 31P. At equilibrium, the amount of 31P that dilutes the 33P added defines the size of the exchangeable inorganic P pool. Inorganic P will be released from the organic P pool by mineralisation to further dilute the 33P added. The rate at which the 33P in the exchangeable inorganic P pool is diluted is termed as the influx rate (Fin), while removal of 31P and 33 P constitute the out-flux rate (Fout). The Fin will be equal to Fout for soils having P status at steady state (i.e. Fin = Fout = flux rate (FR)), and the size of exchangeable inorganic P pool (Q) also
183 remains constant with time (i.e. Q1 = Q2 = Q). Under such conditions, the change in the specific activity in the labelled inorganic P pool follows (Di et al. 2000): Fin ¼ Fout ¼ FR ¼ ½ln SA1 =SA2 Þ:Q=ðt2 t1 Þ ð1Þ In this equation Q is the quantity of readily exchangeable inorganic P in the labelled pool, and SA1 and SA2 are determined at two times after labelling (t1 and t2). To calculate FR using this equation, Q should be constant between t1 and t2. To verify this assumption and quantify the size of readily exchangeable inorganic P pool, Q1 and Q2 are determined at the same time at which the SA1 and SA2 are determined in the labeled soil. The soil samples that are not labelled with 33P are kept under conditions identical to the labelled samples. These ‘unlabelled’ soil samples are shaken with a 33 P solution for the same length of time as the 33P labelled samples. Assuming that this exchange reaches instantaneous completion, the size of readily exchangeable inorganic P pool Q can be calculated using the instSA (33P/31P) and the total 33 P (R) introduced in the unlabelled soil: Q ¼ Q1 ¼ Q2 ¼ R=inst SA
ð2Þ
The flux rates calculated by Equation (1) in the labelled soil at the same Q values at 2 points of time quantifies the gross basal organic P mineralisation when contribution by other P release processes is minimal (Di et al. 2000). Green manure amendment In the absence of a long-term field experiment, green manure ‘conditioning’ of a soil was carried out under glasshouse conditions over a 12 month period. Templeton silt loam soil (Udic ustochrept; 0–10 cm; 23 g organic carbon (C) kg1; 2 g total N kg1; 7 mg Olsen-P kg1; 558 mg total P kg1; P retention capacity 20%) was collected from a field site adjacent to Lincoln University. The green manure, lupin (Lupinus angustifolius, cv. Fest), was grown at the field site for 76 days and harvested prior to the onset of flowering. Approximately 2 kg of soil (dry weight basis) was placed in two plastic containers. One soil sample was kept unamended and the other was amended with fresh lupin (
