The effects of land use change on soil carbon stocks are of concern in the context of international policy agendas on greenhouse gas emissions mitigation. This paper reviews the literature for the influence of land use changes on soil C stocks and reports the results of a meta analysis of these data from 74 publications. The meta analysis indicates that soil C stocks decline after land use changes from pasture to plantation (−10%), native forest to plantation (−13%), native forest to crop (−42%), and pasture to crop (−59%). Soil C stocks increase after land use changes from native forest to pasture (+ 8%), crop to pasture (+ 19%), crop to plantation (+ 18%), and crop to secondary forest (+ 53%). Wherever one of the land use changes decreased soil C, the reverse process usually increased soil carbon and vice versa. As the quantity of available data is not large and the methodologies used are diverse, the conclusions drawn must be regarded as working hypotheses from which to design future targeted investigations that broaden the database. Within some land use changes there were, however, sufficient examples to explore the role of other factors contributing to the above conclusions. One outcome of the meta analysis, especially worthy of further investigation in the context of carbon sink strategies for greenhouse gas mitigation, is that broadleaf tree plantations placed onto prior native forest or pastures did not affect soil C stocks whereas pine plantations reduced soil C stocks by 12–15%.

Soil carbon stocks and land use change: a meta analysis

Soil carbon sequestration to mitigate climate change

Priming effects are strong short-term changes in the turnover of soil organic matter caused by comparatively moderate treatments of the soil. In the course of priming effects large amounts of C, N and other nutrients can be released or immobilized in soil in a very short time. These effects have been measured in many field and laboratory experiments; however, only a few of the studies were aimed at an extended investigation of the mechanisms of such phenomena. The aim of this overview is to reveal possible causes and processes leading to priming actions using the references on agricultural ecosystems and model experiments. Multiple mechanisms and sources of released C and N are presented and summarized in Tables for positive and negative real and apparent priming effects induced after the addition of different organic and mineral substances to the soil. Soil microbial biomass plays the key role in the processes leading to the real priming effects. The most important mechanisms for the real priming effects are the acceleration or retardation of soil organic matter turnover due to increased activity or amount of microbial biomass. Isotopic exchange, pool substitution, and different uncontrolled losses of mineralized N from the soil are responsible for the apparent N priming effects. Other multiple mechanisms (predation, competition for nutrients between roots and microorganisms, preferred uptake, inhibition, etc.) in response to addition of different substances are also discussed. These mechanisms can be distinguished from each other by the simultaneous monitoring of C and N release dynamics; its comparison with the course of microbial activity; and by the labelling of different pools with 14 C or 13 C and 15 N. Quantitative methods for describing priming effects and their dynamics using 14 C and 15 N isotopes, as well as for non-isotopic studies are proposed.

Review of mechanisms and quantification of priming effects.

1 Root Ecology and Root Physiology- 2 General Survey of Root-Study Methods- 21 Historical Development- 22 Principles of Classification- 23 Selection of the Best Method- 3 Excavation Methods- 31 Introduction- 32 Outline of the Classical Method- 321 Selection of the Plant- 322 Digging the Trench- 323 Excavating the Root System- 324 Drawings and Photographs- 325 Preparation and Storage of Excavated Root Systems- 326 Review of Applications- 327 Advantages and Disadvantages- 33 Modifications of the Classical Method- 331 Excavations with Water Pressure- 332 Excavations with Air Pressure- 333 Excavations in a Horizontal Plane- 334 Sector Method- 4 Monolith Methods- 41 Introduction- 42 Simple Spade Methods- 43 The Common Monolith Method- 431 Square Monoliths- 432 Round Monoliths- 44 Box Methods- 45 Cage Methods- 46 Needleboard Methods- 461 General Survey- 462 Construction and Preparation of the Needleboards- 463 Excavating the Monoliths- 464 Washing Procedure- 465 Photographing and Sectioning- 466 Special Modifications- 467 Evaluation of the Methods- 5 Auger Methods- 51 Specific Features- 52 Sampling Techniques- 521 Techniques with Hand Augers- 5211 Sampling Procedure- 5212 Number or Replications- 5213 Special Technique for Studying Tree Roots- 522 Mechanized Techniques- 523 Core-Sampling Machines- 53 The Core-Break Method- 54 Advantages and Drawbacks- 6 Profile Wall Methods- 61 General Survey- 62 The Traditional Trench Profile Method- 621 Digging the Trench- 622 Preparing the Profile Wall- 623 Exposing the Roots- 624 Mapping and Counting Procedure- 6241 Determination of Root Number- 6242 Determination of Root Length- 63 The Foil Method- 64 Technique in a Horizontal Plane- 65 Evaluation and Applications- 7 Glass Wall Methods- 71 Introduction- 72 Glass-Faced Profile Walls- 73 Root Laboratories- 731 General Survey- 732 Features of Their Design- 733 Methods of Recording- 734 Light Sensitivity of Roots- 735 Recent Research in Root Laboratories- 74 Root Observations with Glass Tubes- 75 Evaluation of the Methods- 8 Indirect Methods- 81 Introduction- 82 Determination of Soil Water Content- 821 Gravimetric Method- 822 Neutron Method- 83 Staining Techniques- 84 Uptake of Non-Radioactive Tracers- 85 Radioactive Tracer Methods- 851 General Survey- 852 Soil Injection Technique- 853 Plant Injection Technique- 854 Root Studies with 14C- 855 Critical Evaluation- 9 Other Methods- 91 Measuring Root-Pulling Strength- 92 Measuring Root-Clump Weight- 93 Measuring Root Tensile Strength- 94 Root Measurements Using Soil Sections- 95 Root-Detecting Method- 96 Mesh Bag Method- 97 Root Replacement Method- 98 Root Investigations with Paper Chromatography- 99 Electrical Methods- 910 Determination of Growth Rings in Tree Roots- 911 Investigations of Root Hairs- 912 Determination of Root Nodules- 913 Determination of Mycorrhizae- 10 Container Methods- 101 General Features- 102 Rooting Volume and Container Size- 103 Types of Containers- 1031 Small Pots- 1032 Boxes and Tubes- 1033 Glass-Faced Containers- 1034 Flexible Tubes- 104 Rooting Media- 105 Filling the Containers- 106 Seed Technique- 107 Irrigation Problems- 108 Special Washing Procedures- 109 Modified Container Methods- 1091 Cage Method- 1092 Needleboard Method- 1093 Root Training by Plastic Tubes- 1094 Split-Root Technique- 1095 Undisturbed Soil Monoliths- 1010 Root Studies in Nutrient Solutions- 1011 Root Studies in Mist Chambers- 1012 Comparability of Results from Container Experiments with Field Data- 11 Techniques of Root Washing- 111 Dry Sieving- 112 Storing Soil-Root Samples Before Washing- 113 Chemicals for Facilitating Root Washing- 114 Washing Roots by Hand- 115 Flotation Method- 116 Root-Washing Machines- 117 Nutrient Losses from Roots During Washing- 118 Cleaning Roots from Debris- 119 Storing Roots After Washing- 12 Root Parameters and Their Measurement- 121 General Aspects- 122 Root Number- 123 Root Weight- 1231 Determination of Fresh Weight- 1232 Determination of Dry Weight- 1233 Advantages and Critical Objections- 124 Root Surface- 1241 Calculation from Other Parameters- 1242 Photoelectric Measurements- 1243 Adsorption Methods- 125 Root Volume- 1251 Calculation from Other Parameters- 1252 Displacement Technique- 126 Root Diameter- 1261 Measurements- 1262 Applications in Tree Root Studies- 127 Root Length- 1271 Direct Measurements- 1272 Intersection Methods- 1273 Root-Counting Machines- 1274 Reasons for Increasing Use of Root Length Measurements- 128 Root Tips- 1281 Technique of Counting- 1282 Root Coefficients- 129 Shoot-Root Relations- 13 Some Future Aspects for the Use of Ecological Root-Study Methods- References

Methods of studying root systems

Increasing population pressure is increasing the demand on agricultural systems in many parts of the world and this has often led to the degradation of the soil resource. Soil carbon (C) is a major determinant of sustainability of agricultural systems and changes can occur in both total and active, or labile, C pools. A procedure is presented to determine the degree of lability of soil C. By treating a ground sample of soil with 333 mM potassium permanganate (KMn04) to oxidize a proportion of the carbon and by determining the total carbon by combustion, two fractions of C can be measured. These fractions represent carbon of different lability, with fraction I representing the Labile C (CL), which is oxidized by 333 mM KMn04, and fraction I1 representing the non-labile C (CNL), which is not oxidized by 333 mM KMn04. On the basis of changes in total carbon (CT), a Carbon Pool Index (CPI) is calculated and, on the basis of changes in the proportion of labile C in the soil between a reference site and those subjected to agricultural practice or research treatments, a Lability Index (LI) is determined. These two indices are used to calculate a Carbon Management Index (CMI), with CMI = C Pool Index (CPI) xLability Index (LI) x 100. Analyses of paired samples (cropped and uncropped) from three sites in northern and central New South Wales, Australia, have shown a decline in CPI, a greater decline in LI and hence a decline in the CMI with cropping. Introduction of a legume into a wheat cropping system restored the CMI from 22 to 37 at the Warialda site. Analyses of paired samples from a sugarcane area in north Queensland have shown a decline in CMI in systems dominated by trash burning, but an increase in CMI in systems dominated by green cane trash management. Similar data from Brazil showed no increase in CT with mulching but a 48% increase in CMI due to an increase in the lability of C in the soil. The fractionation procedure and CMI outlined can be used to determine the state and rate of change in soil C of agricultural and natural systems.

Soil Carbon Fractions Based on their Degree of Oxidation, and the Development of a Carbon Management Index for Agricultural Systems

Soil testing for S has generally been unsuccessful when using extractants that remove only sulfate from the soil. An assessment of a range of extractants to predict S status was undertaken on soil samples taken from 18 field trials in northern N.S.W. The extractants were water, 0.01M monocalcium phosphate (MCP) and 0.5M NaHCO3, 0.25 M KCl heated for 3 h at 100, 80, 40, or 25°C. The highest correlation between soil S test level and % maximum yield was found in the 40°C KCl extractant (r2 = 0.73). This compares with an r2 value of 0.47 for the widely used MCP extractant. A study using a soil from a pot experiment where rice was grown showed that the KCl extract removed more S from the HI reducible (ester sulfate) fraction than did MCP. This S fraction is believed to be important in supplying S to plants. A comparison of the specific radioactivity of soil extractants and rice plants confirmed that the KCl 40°C extract removes S from similar soil pools as do plants. The procedure is recommended for wider evaluation.

A soil sulfur test for pastures and crops.

The decline in soil organic matter with cropping is a major factor affecting the sustainability of cropping systems. Changes in total C levels are relatively insensitive as a sustainability measure. Oxidation with different strength KMnO4 has been shown to be a more sensitive indicator of change. The relative size of soil C fractions oxidised by 333mM KMnO4 declined with cropping, whilst the relative size of the unoxidised fraction increased. Changes in δ13 C ratio have been used to measure C turnover in systems which include C3 and C4 species.

Changes in soil organic matter with cropping as measured by organic carbon fractions and 13C natural isotope abundance

The soil survey was conducted on cropped and uncropped Red Earths (Alfisols), Grey Clays, and Black Earths (Vertisols) in northern New South Wales. The degradation of soil physical properties between the cropped and uncropped reference sites was reflected in declines of 29–86% in hydraulic conductivity (K) and 33–71% in aggregation. Generally there was a substantial loss of carbon with cropping, and the loss of labile carbon (CL) was usually greater than the loss of total carbon (CT). A Grey Clay which had been cropped for >40 years had lost 63% and 51% of its CL and CT, respectively. An adjacent Grey Clay which had been cleared and cropped for only 2 years had lost 43% and 26% of its CL and CT, respectively, resulting in a C management index (CMI) of 55, indicating that a large proportion of soil C was lost soon after the commencement of cropping. Where well-managed legume leys had recently been grown, the loss of C was reduced, resulting in a higher CMI. A loss of total and available nutrients after cropping was also found, with the magnitude of the losses modified by fertiliser history. A highly significant correlation was found between CT or CL and the proportion of water-stable aggregates >250 µm for the Red Earth and Grey Clay soils, but this correlation was poor for the Black Earth. The importance of organic matter in maintaining aggregation in low clay soils such as the Red Earth was highlighted.

A survey of the impact of cropping on soil physical and chemical properties in north-western New South Wales

Managing crop residues, fertilizers and leaf litters to improve soil C, nutrient balances, and the grain yield of rice and wheat cropping systems in Thailand and Australia

Graeme J. Blair

Papers

Soil Carbon Fractions Based on their Degree of Oxidation, and the Development of a Carbon Management Index for Agricultural Systems

A soil sulfur test for pastures and crops.

Changes in soil organic matter with cropping as measured by organic carbon fractions and 13C natural isotope abundance

A survey of the impact of cropping on soil physical and chemical properties in north-western New South Wales

Managing crop residues, fertilizers and leaf litters to improve soil C, nutrient balances, and the grain yield of rice and wheat cropping systems in Thailand and Australia