Grasslands are heavily relied upon for food and forage production. A key component for sustaining production in grassland ecosystems is the maintenance of soil organic matter (SOM), which can be strongly influenced by management. Many management techniques intended to increase forage production may potentially increase SOM, thus sequestering atmospheric carbon (C). Further, conversion from either cultivation or native vegetation into grassland could also sequester atmospheric carbon. We reviewed studies examining the influence of improved grassland management practices and conversion into grasslands on soil C worldwide to assess the potential for C sequestration. Results from 115 studies containing over 300 data points were analyzed. Management improvements included fertilization (39%), improved grazing management (24%), conversion from cul- tivation (15%) and native vegetation (15%), sowing of legumes (4%) and grasses (2%), earthworm introduction (1%), and irrigation (1%). Soil C content and concentration in- creased with improved management in 74% of the studies, and mean soil C increased with all types of improvement. Carbon sequestration rates were highest during the first 40 yr after treatments began and tended to be greatest in the top 10 cm of soil. Impacts were greater in woodland and grassland biomes than in forest, desert, rain forest, or shrubland biomes. Conversion from cultivation, the introduction of earthworms, and irrigation resulted in the largest increases. Rates of C sequestration by type of improvement ranged from 0.1 1 to 3.04 Mg C-ha-l yr-l, with a mean of 0.54 Mg C-ha-l yr-l, and were highly influenced by biome type and climate. We conclude that grasslands can act as a significant carbon sink with the implementation of improved management.

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Grassland management and conversion into grassland: effects on soil carbon

Grasslands are heavily relied upon for food and forage production. A key component for sustaining production in grassland ecosystems is the maintenance of soil organic matter (SOM), which can be strongly influenced by management. Many management techniques intended to increase forage production may potentially increase SOM, thus sequestering atmospheric carbon (C). Further, conversion from either cultivation or native vegetation into grassland could also sequester atmospheric carbon. We reviewed studies examining the influence of improved grassland management practices and conversion into grasslands on soil C worldwide to assess the potential for C sequestration. Results from 115 studies containing over 300 data points were analyzed. Management improvements included fertilization (39%), improved grazing management (24%), conversion from cultivation (15%) and native vegetation (15%), sowing of legumes (4%) and grasses (2%), earthworm introduction (1%), and irrigation (1%). Soil C content and concentration increased with improved management in 74% of the studies, and mean soil C increased with all types of improvement. Carbon sequestration rates were highest during the first 40 yr after treatments began and tended to be greatest in the top 10 cm of soil. Impacts were greater in woodland and grassland biomes than in forest, desert, rain forest, or shrubland biomes. Conversion from cultivation, the introduction of earthworms, and irrigation resulted in the largest increases. Rates of C sequestration by type of improvement ranged from 0.11 3.04 Mg C.ha(-1) yr(-1), with a mean of 0.54 Mg C.ha(-1).yr(-1) and were highly influenced by biome type and climate. We conclude that grasslands can act as a significant carbon sink with the implementation of improved management.

Maintenance and improvement of soil quality in continuous cropping systems is critical to sustaining agricultural productivity and environmental quality for future generations. This review focuses on lessons learned from long-term continuous cropping experiments. Soil organic carbon (SOC) is the most often reported attribute from long-term studies and is chosen as the most important indicator of soil quality and agronomic sustainability because of its impact on other physical, chemical and biological indicators of soil quality. Long-term studies have consistently shown the benefit of manures, adequate fertilization, and crop rotation on maintaining agronomic productivity by increasing C inputs into the soil. However, even with crop rotation and manure additions, continuous cropping results in a decline in SOC, although the rate and magnitude of the decline is affected by cropping and tillage system, climate and soil. In the oldest of these studies, the influence of tillage on SOC and dependent soil quality indicators can only be inferred from rotation treatments which included ley rotations (with their reduced frequency of tillage). The impact of tillage per se on SOC and soil quality has only been tested in the ‘long-term’ for about 30 yrs, since the advent of conservation tillage techniques, and only in developed countries in temperate regions. Long-term conservation tillage studies have shown that, within climatic limits: Conservation tillage can sustain or actually increase SOC when coupled with intensive cropping systems; and the need for sound rotation practices in order to maintain agronomic productivity and economic sustainability is more critical in conservation tillage systems than conventional tillage systems. Long-term tillage studies are in their infancy. Preserving and improving these valuable resources is critical to our development of soil management practices for sustaining soil quality in continuous cropping systems.

The role of soil organic matter in maintaining soil quality in continuous cropping systems

(1992). Time Series—A Biostatistical Introduction. Technometrics: Vol. 34, No. 2, pp. 229-230.

Time Series—A Biostatistical Introduction

Many studies carried out during these last few years have focused on the factors influencing plant diversity in species-rich grasslands. This is due to the fact that these ecosystems, among the most diversified in temperate climates, are extremely threatened; in some areas, they have almost disappeared. The re-establishment of these habitats implies to know the living conditions of the associations to be recreated. Very often, the typical species of these communities have become so rarefied that the seed bank or the seed rain are not sufficient to recreate the plant community. Most of the time, to achieve the restoration of these communities, they have to be totally recreated by sowing. For the restoration or the maintenance of the community, the soil chemical characteristics have also to be appropriate or if not modified. This research tends to establish a relation between some soil chemical factors and the plant diversity of a great number of stations. This research has illuminated the relationship between soil extractable phosphorus and potassium and plant diversity. Over 5 mg of phosphorus per 100 g of dry soil (acetate + EDTA extraction), no station containing more than 20 species per 100 m(2) has been found. The highest number of species is found below the optimum content of the soil for plant nutrition (5-8 mg P/100 g). Concerning the potassium, the highest number of species is found at 20 mg/100, a value corresponcing to an optimum content of the soil for plant nutrition. High potassium contents, in opposition to phosphorus contents, are thus compatible with high values of diversity. Other factors (i.e. pH, organic matter, total nitrogen and calcium) do not show so clearly a relation with plant diversity. Excess of N-NO3 is known for its negative effect on the diversity of plant communities. In these environments, apart from the atmospheric deposits which can be important in some areas, N-NO3 is derived mainly from the symbiotic fixation of atmospheric nitrogen by legumes as well as from the mineralization of the organic matter of the soil. It is possible that, when in small quantities, the available soil phosphorus could be a limiting factor of the N-NO3 supply by these two sources. In this hypothesis, nitrogen would remain the main element limitating plant diversity but its availability would be controlled by phosphorus.

Relationship between soil chemical factors and grassland diversity

The effects of varying durations of lucerne ley and subsequent wheat cropping on the moisture status of contrasting wheat growing soils are reported from two experiments in northern New South Wales. Lucerne leys of 5 1/2, 3 1/2 and 1 1/2 years on a red brown earth and 3 1/2, 2 1/2, and 1 1/2 years on a black earth were compared with a wheat-cowpea rotation, long fallow and continuous wheat. Lucerne extracted available soil water within eight months of sowing to a depth of 100 cm, with partial extraction of water to 150 cm. Within 18 months available water to a depth of 200 cm had been extracted. Subsequent growth of lucerne was dependent on incident rainfall and water extraction from depths greater than 200 cm. Wheat extracted water from the soil only to a depth of 150 cm, and extracted less water to this depth than did lucerne. Lucerne extracted water beyond the -1 5 bar suction level. On the black earth, soil moisture at 150 cm had not been recharged before time for sowing the third wheat crop, 28 months after lucerne plough-out. On the red brown earth, after a drought enforced long fallow immediately following lucerne plough-out, soil water at 200 cm was recharged within 18 months. Available soil moisture after three to five months fallow following plough-out of the lucerne leys was much less than for a wheat cropping system, with a consequent severe reduction in the grain yield of the following wheat crop. On the black earth grain yield was reduced by 7.7 kg ha-1 for each reduction of 1 mm in available moisture in the first crop after lucerne plough-out. Data indicate that lucerne must be ploughed out before January if a severe reduction in yield of the following wheat crop is to be minimized.

Effect of grazed lucerne on the moisture status of wheat-growing soils

The effects of pasture improvement on soil pH, total nitrogen, organic carbon and extractable phosphorus (P) were determined by analysing adjacent soils from improved and unimproved pastures at 67 sites on the Northern Tablelands of New South Wales. Pasture improved sites contained at least 1 clover species, predominantly white clover, and had received at least 125 kg P/ha over periods of 15-45 years. The majority of pasture improved sites contained more soil nitrogen, carbon and phosphorus and were of lower soil pH than adjacent unimproved sites. However, the decreases in pH were not statistically significant and not usually related to the magnitude of the increases in other soil fertility parameters nor to the amounts of superphosphate applied or duration of fertiliser history. The largest decline in soil pH and largest increase in organic carbon were on granitic soils which had received more than 250 kg P/ha. The relatively small decreases in soil pH and lack of relationship with fertiliser history, compared with soils from southern New South Wales, were attributed to: (i) re-cycling of legume-fixed nitrogen by summer-growing grasses; (ii) the naturally lower pH, higher nitrogen content and higher buffering capacity of many northern soils. Soil acidification therefore seems to be much slower and less frequent in the perennial pasture systems of the Northern Tablelands of New South Wales.

Effects of pasture improvement with superphosphate on soil pH, nitrogen and carbon in a summer rainfall environment

This study, which commenced in 1921, is the longest running crop rotation experiment in the summer rainfall region of Australia. The 7 rotation treatments comprise various frequencies and combinations of maize and spring oat crops with and without autumn oats and red clover ley. The maize and oat yields are analysed separately using principles described by Paterson (1964). Spline regressions are used to describe the trends of yields with time and to make comparisons amongst the rotations. Yield performance of maize and oats was improved as legume ley duration increased, although as maize cropping became more frequent this effect was reduced. Autumn-sown oats benefited both maize and spring oat yields, independent of the presence of a legume ley. Crop yields were generally maintained in those rotations containing a grazed clover ley. After alteration of ley management in the mid 1960s so that clover crops were subsequently removed as hay, crop yields in these rotations declined. The benefits of clover ley to crop productivity were considered to be primarily due to the maintenance of those soil chemical, physical, and biological properties associated with sustainable crop production.

Rotation effects on sustainability of crop production: the Glen Innes rotation experiment.

Experiments were carried out in the field and glasshouse to determine the requirements for lime and nutrient elements by lucerne growing on moderately acid, weakly buffered granitic soils of the Northern Tablelands of New South Wales. The initial field experiment, which was an omission type at two sites with basal applications of phosphorus and sulfur, gave responses only to molybdenum, whilst on the 'complete' nutrient treatment (potassium, magnesium, molybdenum, manganese, boron, copper, zinc, cobalt and iron), there were large responses to lime. The subsequent glasshouse experiments gave large positive interactions between lime and the complete nutrient treatment and also between lime and phosphorus. The interactions between lime and complete nutrients involved mainly boron and to a lesser extent molybdenum, but at least one other nutrient, probably potassium, was also involved. However, the largest interaction was between lime and phosphorus. There was little indication that the lime effect was caused by increased nutrient availability in the soil or by improved root nodulation; it appeared to be associated with greatly improved root growth. White clover differed from lucerne in giving no interactions between lime and other nutrients and was much less sensitive to lime-induced boron deficiency. The results' of these experiments indicate that lucerne yields on these granitic soils cannot be maximized by the application of nutrient elements unless lime is also applied. Likewise lime has little effect unless deficient nutrients are also applied. Near-maximum yields were obtained with lime at 2.5 t/ha in the presence of adequate levels of deficient nutrients.

Lucerne responses to lime and interactions with other nutrients on granitic soils

The nitrogen, phosphorus, and potassium requirements of sugar cane were studied in relation to the concentration of these elements in the leaf tissue of three varieties of sugar cane grown commercially in Fiji. Percentage yields of sugar cane in fertilizer field experiments were highly correlated with leaf nutrient levels in the control plots, provided leaf sampling was carried out during the maximum growth period of mid- January to mid-May. For each nutrient there was a marginal zone of leaf concentration below which crops always gave significant yield responses to applied nutrients and above which crops failed to respond. Marginal zones for crops sampled during mid-March to mid-May were 1.4-2.0 per cent for nitrogen, 0.13-0.21 per cent for phosphorus, and 0.9-1.5 per cent oven dry leaf for potassium. Within the deficient range of leaf nutrient concentrations there was little relationship between optimum rates of fertilizer required to correct the deficiency and leaf nutrient levels of unfertilized cane. Because of the lateness of sampling, any indication of fertilizer requirement would only be applicable to a subsequent ratoon crop.

Icr Holford

Papers

Effect of grazed lucerne on the moisture status of wheat-growing soils

Effects of pasture improvement with superphosphate on soil pH, nitrogen and carbon in a summer rainfall environment

Rotation effects on sustainability of crop production: the Glen Innes rotation experiment.

Lucerne responses to lime and interactions with other nutrients on granitic soils

Nutrient status of sugar cane in relation to leaf nutrient concentration