Fate of N from Green Manures and Ammonium Sulfate

By means of ^15N tracer technique the fate of N in ammonium sulfate,milk vetch,sesbania and azolla,and the availability of their residual N were studied in a microplot experiment.Results showed that a) at the end of the first crop of early rice,both plant recovery and loss of N from ammonium sulfate were the highest whereas those from azolla were the lowest with those from milk vetch and sesbania in between;the sequence was reversed in terms of recovery of N in soil;the net residual N from ammonium sulfate was very low,about 1/7-1/4 of that from green manures,indicating that chemical N fertilizer contributes little to the soil N reserve;b) plant recovery of the residual N was low and it did not always decrease with time;the total plant recovery (from the second to the fifth crops) of the residual N from various test fertilizers was only 8-11% of the total N originally applied;c) the plant recovery of the residual N from ammonium sulfate was the highest,followed by those from milk vetch and sesbania,and that from azolla was the lowest,no matter in which cropping season (from the second to the fifth);N availability ratio showed the same trend,indicating that chemical N fertilizer helps renovate soil organic N,maintain and increase availability of soil N.     

较贫瘠的红壤中有机质的积累及其生态意义

Field experiments on the decomposition of organic materials and the accumulation of organic carbon in infertile red soils were conducted at the Ecological Experimental Station of Red Soil, the Chinese Academy of Sciences, and the potential of CO₂ sequestration by reclamation and improving the fertility of these soils was estimated. Results showed that in infertile red soils, the humification coefficients of organic materials were rather high, ranging from 0.28 to 0.63 with an average of 0.43, which was 41% higher than those in corresponding red soils with medium fertility. This was mainly attributed to the high clay content, high acidity and low native organic matter content of infertile red soils. Compared to those in corresponding normal red soils, the decomposition rates of organic materials were significantly lower in infertile red soils in the first 2 years, thereafter no significant difference was observed between those in the two kinds of soils. Depending on the kind and amount of organic manure applied, the soil properties and the rotation systems, annual application of organic manure with a rate of 4 500 to 9 000 kg ha^-1 increased the organic carbon content in surface 20 cm of infertile red soils by 2.1~7.5 g kg^-1 with an average of 4.7 g kg^-1 within the first 5 years. The organic carbon content in infertile red soils which received organic manure annually increased linearly in the first 10 years, thereafter it slowed down, implying that the fertility of the infertile red soils could reach middle or high level in 10 years if the soil was managed properly. It was estimated that through exploitation of wastelands, re-establishment of fuel forests and improvement of soil fertility, soils in red soil region of China could sequester an extra 1.50 × 10¹⁵ g of atmospheric CO₂.     

Mineralization and Transformation of Nitrogen Derived from Plant Materials in Soils over 10 Years

Results of a 10-year decomposition experiment indicated that the annual mineralization rate of organic N in newly-formed humus varied with the type of original plant materials and the water regimes for decomposition,ranging from 0.028 to 0.074.The mineralization rate under waterlogged conditions was higher than that under upland conditions.The proportion of α-amino acid N in humus newly-formed under waterlogged conditions was slightly higher than that under upland conditions.It decreased gradually with time,while the proportion of nonhydrolyzable N showed no consistent trend,irrespective of the water regines for decomposition.The distribution of amino acids in humus newly-formed from different plant materials under various water regimes was quite similar with that in original plant materials,and only minor differences could be found among them.For example,in comparison to original plant materials,the newly-formed humus contained higher proportions of isoleucine,cysting,γ-amino-butyric acid and ornithine,and lower proportions of phenylalanine and proline.Moreover the proportion of phenylalanine was higher in the humus newly-fored under waterlogged conditions than that under upland conditions.     

中国亚热带稻田土壤碳氮含量及矿化动态

Dynamics of soil organic matter in a cultivation chronosequence of paddy fields were studied in subtropical China. Mineralization of soil organic matter was determined by measuring CO2 evolution from soil during 20 days of laboratory incubation. In the first 30 years of cultivation, soil organic C and N contents increased rapidly. After 30 years, 0-10 cm soil contained 19.6 g kg^-1 organic C and 1.62 g kg^-1 total N, with the corresponding values of 18.1 g kg^-1 and 1.50 g kg^-1 for 10-20 cm, and then remained stable even after 80 years of rice cultivation. During 20 days incubation the mineralization rates of organic C and N in surface soil （0-10 cm） ranged from 2.2% to 3.3% and from 2.8% to 6.7%, respectively, of organic C and total N contents. Biologically active C size generally increased with increasing soil organic C and N contents. Soil dissolved organic C decreased after cultivation of wasteland to 10 years paddy field and then increased. Soil microbial biomass C increased with number of years under cultivation, while soil microbial biomass N increased during the first 30 years of cultivation and then stabilized. After 30 years of cultivation surface soil （0-10 cm） contained 332.8 mg kg^-1 of microbial biomass C and 23.85 mg kg^-1 of microbial biomass N, which were 111% and 47% higher than those in soil cultivated for 3 years. It was suggested that surface soil with 30 years of rice cultivation in subtropical China would have attained a steady state of organic C content, being about 19 g kg^-1.     

Effects of Soil Texture and CaCO3 on Turnover of Organic Material in Chao Soils

Decomposition experiments of ^14C-labelled sickle alfalfa in chao soils of different texture and these soils after removal of CaCO3 were carried out under field and laboratory conditions respectively.The amount of residual ^14C in,or ^14CO2 evolved from,the soils at intervals after the beginning of devomposition were measured and the distribution of native and labelled C between particle size fractions isolated from these soils was edtermined.Results showed that contents of both labelled (^14C) and non-labelled (^12C) carbon decreased with increasing particle size.The enrichment factor for ^14C was higher than that for ^12C in the clay fraction,the reverse being true for the silt enrichment factors.The effect of soil texture on the decomposition of plant material could not be observed in chao soils when the clay content was lower than 270g kg^-1,while it became obvious once CaCO3 was removed was correlated from these soils.The decomposition rate of plant material in the soil from which the native CaCO3 Was removed was correlated significantly to both the clay content of the soil and the application rate of CaCO3.A preliminary correction equation describing the effect of clay and CaCO3 on the decomposition of organic material in chao soil was derived from the results obtained.     

Characteristics of Humic Substances in Paddy Soils

In the present paper,the composition of humus and the charateristics of humic acid from seven paddy soils were compared with those of upland (and/or natural) soils.Results show that:(1) in each group of the soil samples for comparison the HA/FA ratio of the humus of a paddy soil,in most cases,was appreciable higher than that of adjacent upland(and/or natural) soil derived from the same parent material;(2) the humic acid extracted from the paddy soils was characterized by a higher C/O ratio,a higher content of methoxyl groups,and a lower content of carboxyl groups than those from the corresponding upland (and/or natural) flooded soils,implying that the humic acid formed under rice cultivation is in a lower degree of humification than that formed under upland(and/or natural) conditions;and (3) the humic acid of paddy soils,however,was not always characterized by a lower aromaticity than that of the corresponding upland(and/or natural) soils.     

尾矿池里添加大理石废料和有机物五年后植被覆盖和土壤生化特性研究

Tailing ponds pose environmental hazards,such as toxic metals which can contaminate the surroundings through wind and water erosions and leaching.Various chemical and biochemical properties,together with extractable and soluble metals were measured five years after reclamation of a polluted soil affected by former mining activities.This abandoned mine site contains large amounts of Fe-oxyhydroxides,sulphates,and heavy metals.As a consequence,soils remain bare and the soil organic matter content is low(< 3 g kg 1).Marble waste,pig manure and sewage sludge were applied in 2004.Plant cover and richness,and soil chemical,biochemical and biological parameters were analysed five years later.Results showed that all soil biochemical properties as well as vegetation cover and richness were higher in treated soils than in the untreated contaminated plots(control),although organic matter,pH values and extractable metals concentrations were similar among treatments.Soluble cadmium and zinc were lower in the amended plots than in control.As a general pattern,soil amended with pig manure showed higher values of most biochemical properties compared to sewage sludge application,while the doses did not have a great effect,being only significant for β-glucosidase,phosphodiesterase and arylsulfatase.Significant correlations were found between vegetation cover,richness and soil biochemical properties,suggesting a high interdependence between plant colonization and reactivation of biogeochemical cycles during five years.This study confirms the high effectiveness of an initial application of the amendments tested to initialize the recovery of ecosystems in bare mine soils under Mediterranean semiarid conditions.This research also shows the high sensitivity of certain biochemical properties in order to evaluate soil quality and reactivation of nutrients cycles in reclaimed mine soils.     

利用方式和土壤肥力对土壤团聚体和养分的影响

The size distribution of water-stable aggregates and the variability of organic C, N and P contents over aggregate size fractions were studied for orchard, upland, paddy, and grassland soils with high, medium, and low fertility levels. The results showed that > 5 mm aggregates in the cultivated upland and paddy soils were 44.0% and 32.0%, respectively, less than those in the un-tilled orchard soil. Organic C and soil N in different size aggregate fractions in orchard soil with high fertility were significantly higher than those of other land uses. However, the contents of soil P in different size aggregates were significantly greater in the paddy soil as compared to the other land uses. Soil organic C, N and P contents were higher in larger aggregates than those in smaller ones. The amount of water-stable aggregates was positively correlated to their contribution to soil organic C, N and P. For orchard and grassland soils, the > 5 mm aggregates made the greatest contribution to soil nutrients, while for upland soil, the 0.25-0.053 mm aggregates contributed the most to soil nutrients. Therefore, the land use with minimum disturbance was beneficial for the formation of a better soil structure. The dominant soil aggregates in different land use types determined the distribution of soil nutrients. Utilization efficiency of soil P could be improved by converting other land uses to the paddy soil.     

An Invasive Succulent Plant (Kalanchoe daigremontiana) Influences Soil Carbon and Nitrogen Mineralization in a Neotropical Semiarid Zone

It has long been recognized that plant invasions may alter carbon(C) and nitrogen(N) cycles, but the direction and magnitude of such alterations have been rarely quantified. In this study, we quantified the effects caused by the invasion of a noxious exotic plant,Kalanchoe daigremontiana(Crassulaceae), on C and N mineralization and enzymatic and microbial activities in the soil of a semiarid locality in Venezuela. We compared soil parameters associated with these processes(C and N mineralization time and the cumulative values, fluorescein diacetate hydrolytic activity, and activities of dehydrogenase, β-glucosidase, glucosaminidase, and urease) between invaded and adjacent non-invaded sites. In addition, correlations among these parameters and the soil physical-chemical properties were also examined to determine if a positive feedback exists between nutrient availability and K. daigremontiana invasion. Overall,our results showed that C mineralization and transformation of organic compounds to NH_4~+ were favored at sites colonized by K.daigremontiana. With this species, we found the highest cumulative amounts of NH_4~+-N and C and the lowest mineralization time.These results could be explained by higher activities of urease and glucosaminidase in soils under the influence of K. daigremontiana.In addition, higher amounts of organic matter and moisture content in invaded soils might favor C and N mineralization. In conclusion,invasion of Neotropical semiarid zones by K. daigremontiana may influence the chemical and biological properties of the soils covered by this species, increasing nutrient bioavailability, which, in time, can facilitate the invasion process.     

施用液态家畜废物的水稻田土壤中硝酸盐异化还原成铵过程的研究

Most studies on dissimilatory nitrate reduction to ammonium (DNRA) in paddy soils were conducted in the laboratory and in situ studies are in need for better understanding of the DNRA process.In this study,in situ incubations of soil DNRA using 15 N tracer were carried out in paddy fields under conventional water (CW) and low water (LW) managements to explore the potential of soil DNRA after liquid cattle waste (LCW) application and to investigate the impacts of soil redox potential (Eh) and labile carbon on DNRA.DNRA rates ranged from 3.06 to 10.40 mg N kg 1 dry soil d 1,which accounted for 8.55%-12.36% and 3.88%-25.44% of consumption of added NO 3-15 N when Eh at 5 cm soil depth ranged from 230 to 414 mV and 225 to 65 mV,respectively.DNRA rates showed no significant difference in paddy soils under two water managements although soil Eh and/or dissolved organic carbon (DOC) were more favorable for DNRA in the paddy soil under CW management 1 d before,or 5 and 7 d after LCW application.Soil DNRA rates were negatively correlated with soil Eh (P < 0.05,n=5) but positively correlated with soil DOC (P < 0.05,n=5) in the paddy soil under LW management,while no significant correlations were shown in the paddy soil under CW management.The potential of DNRA measured in situ was consistent with previous laboratory studies;and the controlling factors of DNRA in paddy soils might be different under different water managements,probably due to the presence of different microfloras of DNRA.     

Changes in microbial biomass after continuous application of azolla and rice straw in soil

A model experiment was conducted under tropical conditions with a view to evaluating the changes in microbial biomass and nutrient dynamics in upland soil through the continuous application of azolla and rice straw (2 g C kg^-1 soil per each application). Flush decomposition of C was observed immediately after each application and the rate of mineralization did not change appreciably during this period. After flush decomposition, the rate of C mineralization from azolla was higher than that from rice straw until 9 to 13 weeks after each application and thereafter the mineralization rate was similar. The amount of inorganic N released from azolla increased following each application, whereas inorganic N in rice straw plot was immediately immobilized and the rate of immobilization increased until the 3rd application and did not increase further after the 4th application. The amounts of biomass C and N increased immediately after residue incorporation, reached the maximum level one week after each application and declined thereafter. Maximum biomass formation increased until the 2nd application and then the level remained constant. Maximum biomass N formation was higher in azolla than in rice straw after the 1st application, but after repeated applications, the difference became less pronounced: Continuous increase in biomass in a certain week after each application was observed, probably because of the cumulative effects of the previous applications. The increase suggests that continuous application of organic materials may enable to improve the amount of soil microbial biomass.     

Ecological study on the dynamics of soil organic matter and its related properties in shifting cultivation systems of Northern Thailand

Abstract

There is a large number of hill people in northern Thailand, who practices shifting cultivation. In order to analyze the soil ecological problems involved in the transition from traditional shifting cultivation to more intensive upland farming, the authors carried out comparative studies on the dynamics of organic matter and its related properties in soils both in the traditional shifting cultivation systems adopted by Karen people and more intensive upland farming practiced by Thai and Hmong people in the area. The contents of organic matter and available N in the surface 10 cm layers of soil from the fields continuously cultivated were lower than those in soils under prolonged fallow (more than 10 y) or natural forest. Based on the rate of soil respiration, the amount of organic matter decomposed within 1 y was estimated to reach nearly 10% of that stored in the upper 50 cm layers of the soil profile in the upland crop fields. These results indicate that the organic matter-related resources markedly decreased under continuous cropping. The contents of C, N, and P in the microbial biomass of the surface 10 cm layers of soil ranged from 0.37 to 2.09 mg C g^?l soil, from 22.7 to 188 µg N g^?l soil, and from 6.1 to 65.7 µg P g^?l soil, respectively. Since the contents of microbial C, N, and P in the surface soils were generally higher under prolonged fallow and natural forests than in the fields continuously cultivated, the microbial activity and/or the amounts of C, N, and P available for biological activity seemed to have declined under continuous upland farming. The incubation experiment to assess the N mineralization pattern showed two remarkable characteristics: 1) there was an initial time lag until active mineralization of N occurred in the soils from young fallow forest and 2) the soil burning effect was observed after burning in the fields under prolonged fallow. The active process of nitrification after N mineralization was always associated with a sharp fall in soil pH, suggesting that soil acidification was promoted and basic cations were lost from the soils. In conclusion, rapid deterioration of the soil organic matter-related properties in cropping fields can be considered to be one of the ecological reasons why upland fields must be returned to fallow again a few years after forest reclamation in traditional shifting cultivation systems. Therefore, in alternative farming systems with more intensive land use, it is essential to apply organic materials into soils to decrease the rate of soil degradation, or to improve the soil fertility, in avoiding soil acidification along with nitrification.     

Turnover of carbon and nitrogen through the microbial biomass in a sandy loam and a clay soil incubated with [14C(U)]glucose and [15N](NH4)2So4 under different moisture regimes

Two soils, one a sandy loam and the other of relatively high clay content, were incubated with [¹⁴C(U)]gtucose and [¹⁵N](NH₄)₂SO₄ for 101 days, either under continuously moist conditions, or with intermittent drying of soils. Rates of evolution of ¹⁴CO₂, decline in residual organic ¹⁴C, and net immobilization and mineralization of N and ¹⁵N in the sandy loam soil were more rapid than in the clay soil. First order decay rates for the decomposition of residual ¹⁴C, after 10 days, were consistently twice as fast in the sandy loam soil. By contrast, the efficiency with which glucose was utilized within the first few days, and the amounts of C, ¹⁴C, N and ¹⁵N present as soil biomass throughout the incubation, were greater in the clay soil than in the sandy loam. Biomass ¹⁴C as a percentage of residual organic ¹⁴C, was consistently 1.5 times greater in the clay soil. Compared with soils held continuously moist, soils which were intermittently dried and remoistened contained smaller amounts of isotope-labelled biomass C and N, but overall similar amounts of total residual organic ¹⁴C and ¹⁵N. Remoistening of dried soils caused a temporary (4 days) flush in C and N mineralization rates.A simulation model describes C and N behaviour in the two soils. Three features of the model are proposed to expain short-term differences between soils in the rates of C and N turnover, viz. the clay soil (a) has a greater capacity to preserve biomass C and N (b) holds a higher proportion of microbial decay products in the near vicinity of surviving cells, and, to a lesser extent, (c) utilizes glucose and metabolic products more efficiently for biosynthetic reactions.     

Mineralization and microbial biomass formation in upland soil amended with some tropical plant residues at different temperatures

A model experiment was carried out at 15, 25, and 35°C to investigate the changes in microbial biomass and the pattern of mineralization in upland soil during 8 weeks following the addition of 8 organic materials including 6 tropical plant residues, ipil ipil (Leucaena leucocephala), azolla (Azolla pinnata), water hyacinth (Eichhornia crassipes), dhaincha (Sesbania rostrata), cowpea (Vigna unguiculata), and sunhemp (Crotalaria juncea). The amounts of CO₂-C evolved and inorganic N produced at 35°C were about 2 times larger than those at 15°C. At any temperature, the flush decomposition of C was observed within the first week and thereafter the rate of mineralization became relatively slow. A negative correlation was observed between inorganic N and C/N ratios of the added organic materials. The relationships between the amounts of cellulose or cellulose plus hemicellulose and the amount of mineralized N of the added organic materials were also negative.

The changes in the microbial biomass were affected by temperatures. The amount of biomass C and N was maximum after 42 d of incubation at 15°C, and after 7 d at 25 and 35°C, and thereafter decreased. The rate of biomass decline was slower at 15°C and faster at 35°C than at 25°C. Regardless of the temperatures, the addition of organic materials enhanced microbial biomass formation throughout the incubation periods.     

Uptake of carbon and nitrogen through roots of rice and corn plants,grown in soils treated with 13C and 15N dual-labeled cattle manure compost

Nitrogen and carbon dynamics in paddy and upland soils for rice cultivation and in upland soil for corn cultivation was investigated by using ¹³C and ¹⁵N dual-labeled cattle manure compost (CMC). In a soil with low fertility, paddy and upland rice took up carbon and nitrogen from the CMC at rates ranging from 0.685 to 1.051% of C and 17.6–34.6% of N applied. The ¹³C concentration was much higher in the roots than in the plant top, whereas the ¹⁵N concentration differed slightly between them, indicating that organic carbon taken up preferentially accumulated in roots. The ¹³C recovery in the plant top tended to be higher in upland soil than in paddy soil, whereas ¹⁵N applied was recovered at the same level in both paddy and upland soils. In the experiment with organic farming soil, paddy rice took up C and N from the CMC along with plant growth and the final recovery rates of ¹³C and ¹⁵N were 2.16 and 17.2% of C and N applied. In the corn experiment, a very large amount of carbon from the CMC was absorbed, accounting for at least 7 times value for rice. The final uptake rates of ¹³C and ¹⁵N reached about 13 and 10% of C and N applied, respectively. Carbon emission from the CMC sharply increased by 2 weeks after transplanting and the nitrogen emission was very low. It is concluded that rice and corn can take up an appreciable level of carbon and nitrogen from the CMC through roots.     

Net Nitrogen and Sulfur Mineralization in Mountainous Soils Amended with Indigenous Plant Residues

Abstract

Mineralization of nitrogen (N) and sulfur (S) were examined over a 31‐week incubation period under aerobic conditions in 13 soils selected from the mountainous district of Pertouli, central Greece. The main native plant species are turf, forage crops, forest trees, herbs, and shrubs. Net mineralization and immobilization of N and S by indigenous plants were estimated. Most soils are acidic in this xeric, mesic climatic regime, and organic carbon (C), organic nitrogen (N), and sulfur (S) appreciably varied in soils and plant species. The cumulative net mineralization of N is much higher in comparison to the amount of mineralized S. Release of N is curvilinear with time and associated with a declining mineralization in the later stages of the incubation. Close relationships exist between soil organic S and N and between organic C and N, indicating that these elements are mainly bound in the organic matter. The cumulative net mineralization from soils varies greatly and ranges between 36.6 and 212.8 (average 104.8) mg/kg for N and between 21.4 and 45.2 (average 31.8) mg/kg for S. Immobilization occurs in most soils amended with indigenous plant residues, with the amount of immobilized N varying between 14.8 and 49.5 mg/kg and that of immobilized S ranging from 2.6 to 30.7 mg/kg. However, the estimated rates of N and S mineralization are not negligible and can be taken into account as potential sources in the management of the upland soils.     

Mineralization of physically fractionated rotten plant residues under upland conditions

A well-rotten mixture of rice straw and calcium cyanamide (rice straw compost prepared indoors) was separated physically into four fractions using a combination of methods which involved sieving, sedimentation and centrifugation. The four fractions obtained were Fl-3 (>0.043 mm), F4 (sedimented at 4°C), F5 (sedimented by centrifugation at 10⁴ × g for 10 min) and F6 (supernatant). The air-dried and pulverized fractions were mixed with upland soils and incubated for 4 weeks at 30°C under upland conditions. The amount of nitrogen mineralized in each fraction was determined, In a red yellow soil, 11.2% of the organic nitrogen present in the unfractionated, air-dried sample was mineralized, compared with 6.8% in volcanic ash soil. The contribution of fraction F5 to the total amount of mineralized nitrogen in wet compost was the highest, followed by F6. Fraction Fl-3 showed immobilization of inorganic nitrogen in both soil types. On the other hand, fractionated samples obtained after air-drying the wet compost showed no immobilization for Fl-3, although the values obtained for other fractions were similar to those obtained for the corresponding fractions of wet compost.

Well-rotten plant residues such as rice straw (compost prepared outdoors), Timothy and Ladino clover were air-dried, then separated into fractions and analyzed for elementary composition as well as inorganic nitrogen content using the same procedures. Although the amounts of mineralized nitrogen were higher in fractions F5 and F6 compared with other fractions, the values were much lower compared with those of undecomposed plant residues.

It was found that the amount of organic nitrogen mineralized in soil was affected not only by the C/N ratio of the plant residues but also by the differences in characteristics or properties of the plant materials and soils.     

Testing the Rothamsted Carbon Model against data from long-term experiments on upland soils in Thailand

We tested the Rothamsted Carbon Model (RothC) against three long‐term (27–28 years) experimental sites on Thai upland soils in order to see how this widely used ‘temperate’ soil carbon turnover model performed in a typical farming region in the tropics. We were able to verify – over a much longer period than had been examined in previous studies – that RothC performs well in a tropical region in plots used for continuous cropping experiments of maize and cassava without organic matter application. However, the model overestimated soil organic carbon (SOC) in some plots to which large amounts of organic matter (rice straw or cassava stalks) were applied. This overestimate could not be attributed to errors in estimating either the amount of C input to the soil or the ratio of decomposable plant materials to resistant plant materials entering the soil. Among many factors affecting SOC dynamics (e.g. weather conditions, soil characteristics, etc.), which are different in tropical regions from temperate regions, we conclude that the activity of soil fauna might be a major factor which makes the performance of RothC worse where much organic matter was applied. We suggest that care should be taken when applying RothC to tropical soils with large amounts of added organic matter.     

Microbial biomass formed from 14C, 15N-labelled plant material decomposing in soils in the field

The decomposition of ¹⁴C, ¹⁴N-labelled medic (Medicago littoralis) material and the net formation and decay of isotope-labelled biomass have been measured in four South Australian soils in the field over 4 yr. The field sites were in similar climatic zones but two sites received about twice as much rainfall as the others. The soils were calcareous and of similar pH, but differed in texture and organic matter content. The decomposition of the organic-¹⁴C and organic-¹⁵N residues were, for a given site, similar. Initially, the concentrations of labelled residues decreased rapidly, then very slowly. Decomposition rates in a heavy clay soil were significantly less than in the other soils during the first 16 weeks after incorporation of plant material, but thereafter, rates of decomposition in all soils were similar, despite differences in soil texture and climate. More than 50% of the medic-¹⁴C had disappeared from all soils after 4 weeks of decomposition and only 15–20% of the medic-¹⁴C remained as organic residues after 4 yr. Of the medic-¹⁵N 60–65% remained as organic residues after 32 weeks decomposition; the percentage decreased to 45–50% after 4 yr.The amounts of ¹⁴C, ¹⁴N-labelled biomass, formed from decomposing plant material, were maximal 4–8 weeks after incorporation of plant material into the soils. In samples taken at 8 weeks from the sandy Roseworthy soil, biomass-¹⁴C and -¹⁵N accounted for 14 and 22% respectively of the total organic-¹⁴C and -¹⁵N residues present. Thereafter in this soil, the concentrations of biomass-¹⁴C and -¹⁵N decreased, rapidly at first then more slowly. Nevertheless, throughout most of the decomposition the rates of decrease in the concentrations of biomass-¹⁴C and -¹⁵N exceeded those of the non-biomass, labelled organic residues.The proportions of ¹⁴C, ¹⁵N-labelled materials accounted for in the labelled biomass varied between soils. Soils of higher clay content generally retained higher proportions of residual organic-¹⁴C and -¹⁴N in the biomass, even though the net rates of decomposition of total labelled residues did not differ significantly between soils during most of the decomposition.