Calculating net primary production and annual input of organic matter to soil from the amount and radiocarbon content of soil organic matter

Soil samples collected from four of the Rothamsted long-term field experiments over the last 100 yr were used to follow the effects of management on soil organic matter content. The experimental sites were:Broadbalk and Geescroft Wildernesses, both on old arable land that had been fenced off in the early 1880s and allowed to revert naturally to deciduous woodland; the unmanured plot in the Rothamsted Park Grass Continuous Hay Experiment, started in 1856; the unmanured and one of the NPK plots in the Broadbalk Continuous Wheat Experiment started in 1843. Total C, radiocarbon and (in some cases) soil microbial biomass C were measured in stored and contemporary soil samples. The current Rothamsted model for the turnover of organic C in soil was then used to calculate how the organic C content of the topsoils from the four sites would change with time for a range of annual inputs. The inputs that generated the best fit to the measured values were: Broadbalk Wilderness 3.5 t C ha−1 yr−1; Geescroft Wilderness 2.5 t C ha−1 yr−1;unmanured plot on Park Grass 3.01 t C ha−1 yr−1; unmanured plot on Broadbalk Wheat 1.3 t C ha−1 yr−1; NPK plot on Broadbalk Wheat 1.71 t C ha−1 yr−1. The model also generated the radiocarbon content of soil organic C for these inputs of C, knowing the 14C content of the atmosphere over the period 1840–1985. The marked increase in the 14C content of soil organic C brought about by thermonuclear testing during the 1960's was accurately represented by the model. The quantities of soil microbial biomass (measured by fumigation-incubation) in the top 23 cm of soil from the four sites were: Broadbalk Wilderness,1.571 t C ha−1; Geescroft Wilderness, 0.58 t C ha−1; unmanured plot on Park Grass,1.621C ha−1; unmanured plot on Broadbalk Wheat, 0.47 t C ha−1; NPK plot on Broadbalk Wheat, 0.76 t C−1.The values for soil microbial biomass C generated by the model for the above annual inputs agreed closely (with one exception) with the measured values. For a site under steady-state conditions, the annual input of organic matter to the soil plus the annual removal of organic matter from the site (if any) gives the Net Primary Production (NPP). NPP thus calculated was 4.0 t C ha−1 yr−1 for the unmanured plot on Park Grass, 2.2 for the unmanured plot on Broadbalk and 5.2 for the NPK plot on Broadbalk. The two Wilderness sites are still accumulating C in both soil and vegetation and here NPP is given by annual input to the soil, plus the annual increment of C in the trees. NPP calculated in this way was 4.8 t C ha−1 yr−1 for Broadbalk Wilderness and 3.3 for Geescroft Wilderness. This new way of estimating NPP, from measurements made on soil organic matter, needs to be tested on a wider range of soils, climates and vegetation types before it can be generally recommended. However, it has many potential advantages, not least that it can give a value for NPP that is integrated over many years from a single sampling. For sites under steady-state conditions it is not essential to have stored soil samples—the necessary measurements can be made on contemporary samples alone.

[1]  A. Johnston,et al.  Modern developments in an experiment on permanent grassland started in 1856 - effects of fertilizers and lime on botanical composition and crop and soil analyses , 1976 .

[2]  L. F. Curtis,et al.  The Soils and Land Use of the District around Aylesbury and Hemel Hempstead , 1965 .

[3]  P. Brookes,et al.  Seasonal changes of soil microbial biomass in an arable and a grassland soil which have been under uniform management for many years , 1990 .

[4]  Knute J. Nadelhoffer,et al.  Belowground Carbon Allocation in Forest Ecosystems: Global Trends , 1989 .

[5]  D. Jenkinson,et al.  Model estimates of CO2 emissions from soil in response to global warming , 1991, Nature.

[6]  D. Jenkinson,et al.  Modelling the turnover of organic matter in long-term experiments at Rothamsted , 1987 .

[7]  A. Johnston,et al.  The Park Grass Experiment , 1964 .

[8]  E. Paul,et al.  ORGANIC CARBON DYNAMICS IN GRASSLAND SOILS. 1. BACKGROUND INFORMATION AND COMPUTER SIMULATION , 1981 .

[9]  W. Parton,et al.  Dynamics of C, N, P and S in grassland soils: a model , 1988 .

[10]  D. Jenkinson,et al.  The nitrogen cycle in the broadbalk wheat experiment: A model for the turnover of nitrogen through the soil microbial biomass , 1989 .

[11]  P. Barraclough,et al.  The Effects of Prolonged Drought and Nitrogen Fertilizer on Root and Shoot Growth and Water Uptake by Winter Wheat , 1989 .

[12]  P. Brookes,et al.  Microbial biomass measurements in forest soils: The use of the chloroform fumigation-incubation method in strongly acid soils , 1987 .

[13]  W. Parton,et al.  Analysis of factors controlling soil organic matter levels in Great Plains grasslands , 1987 .

[14]  J. Lynch,et al.  The turnover of organic carbon and nitrogen in soil. , 1990 .

[15]  D. Jenkinson.  STUDIES ON THE DECOMPOSITION OF PLANT MATERIAL IN SOIL , 1965 .

[16]  D. Powlson,et al.  The nitrogen cycle in the Broadbalk Wheat Experiment: recovery and losses of 15N-labelled fertilizer applied in spring and inputs of nitrogen from the atmosphere , 1986, The Journal of Agricultural Science.

[17]  R. T. Watson,et al.  Greenhouse gases and aerosols , 1990 .

[18]  D. Jenkinson.  The turnover of organic matter in soil. , 1968, The Biochemical journal.

[19]  A. Walton,et al.  Fluctuations of atmospheric carbon-14 concentrations during the past century , 1971, Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences.

[20]  P. Brookes,et al.  AN EXTRACTION METHOD FOR MEASURING SOIL MICROBIAL BIOMASS C , 1987 .

[21]  David S. Powlson,et al.  The effects of biocidal treatments on metabolism in soil—V: A method for measuring soil biomass , 1976 .

[22]  J. Southon,et al.  AMS 14C Measurements of Fractionated Soil Organic Matter: An Approach to Deciphering the Soil Carbon Cycle , 1989, Radiocarbon.

[23]  D. Jenkinson,et al.  A comparative study of titrimetric and gravimetric methods for the determination of organic carbon in soil , 1973 .

[24]  D. Jenkinson.  STUDIES ON THE DECOMPOSITION OF PLANT MATERIAL IN SOIL. V. THE EFFECTS OF PLANT COVER AND SOIL TYPE ON THE LOSS OF CARBON FROM14C LABELLED RYEGRASS DECOMPOSING UNDER FIELD CONDITIONS , 1977 .

[25]  W. E. Brenchley,et al.  Recolonisation of Cultivated Land Allowed to Revert to Natural Conditions , 1915 .

[26]  Rothamsted Experimental Station Uk.  Rothamsted Experimental Station Report for 1979 Part 1. , 1969 .

[27]  D. S. Jenkinson,et al.  THE TURNOVER OF SOIL ORGANIC MATTER IN SOME OF THE ROTHAMSTED CLASSICAL EXPERIMENTS , 1977 .

[28]  H. Scharpenseel,et al.  Radiocarbon dating of soils, a review , 1977 .

[29]  T. Meyers,et al.  Measuring Biosphere‐Atmosphere Exchanges of Biologically Related Gases with Micrometeorological Methods , 1988 .

[30]  B. J. O'brien.  Soil organic carbon fluxes abd turnover rates estimated from radiocarbon enrichments , 1984 .