Land restoration: a neglected task

H.W. Scharpenseel Land restoration: a neglected task There is much talk of the ecological disasters which result from clearing land by slash and burn: lowering of the water table, spreading savannah colonized by Imperata cylindrica, and desertification. To date, however, few determined efforts have been made to restore degraded soils so that they can be used for sustained agriculture, thereby preserving forest reserves which would otherwise have to be cleared. Of the remaining one billion or so hectares of tropical rainforest, land clearing operations consume from 7 to 15 million ha per year. The burning of woodland for shifting cultivation and land rotation annually adds about 1.5 billion t of carbon to the total atmospheric carbon pool of 700 billion t. Combined with the 5.5 billion tonnes released annually from burning fossil fuels, this amounts to 1% of the total carbon pool. After a 50 year period of mixing in the sea, this has led to an increase of nearly 20% in atmospheric CO2 content from pre-industrial times to the present. This is another reason why clearing land by slash and burn is ecologically unsound. Avoiding degradation Every year about 115 billion t of carbon are converted by photosynthesis. Most of this carbon is absorbed by the 4 billion ha of woodland, with proportionally less by the 1.5 billion ha of agricultural land. Of the total carbon converted annually, less than 2 billion t are required for human nutrition. At present, most of the land used for agriculture has only moderate yields and human nutritional needs are not being met. At great expense, a maximum of nearly 4 billion ha could be made available for food production but this would mean sacrificing much of the 4 billion ha of woodland. A better alternative would be to increase the yield of existing agricultural land. This can only be done if further land degradation can be avoided and already degraded land restored If slash and burn is an ecologically unsound way of clearing land, what are the alternatives? The mechanized clearing of land by crawler tractors, bulldozers and tree pushers is particularly destructive. It promotes erosion, causes extreme soil compaction, and reduces the value of the topsoil and nutrient resources by digging up subsoil and raw sediments. This in turn leads to further nutrient losses, especially in the acidic low activity clay soils of tropical rain forests. Such soils then need medium inputs of nutrients, and after a few years settlers often leave the land in desperation because of the dwindling crop returns. A few years later the conversion of this land into wasteland, such as Imperata cylindrica savanna, is complete. Manual land clearing with motor saws is slower but is less harmful ecologically. No raw sediment is brought to the surface and soil compaction is minimal. If immediately after clearing a mulch is applied, or a cover of Puearia legumes (tropical kudzu) is sown, erosion will be low and the chances for an ecologically sustainable crop culture are good. Ideally, tree crops like coconut, rubber, oil palm, cocoa, etc. are planted in small areas where the kudzu is removed. Strip culture can also be successful. The soil is enriched with nitrogen by the diazotrophic system of the kudzu but often requires some rough phosphate to sustain good yields. With this type of clearing, as opposed to mechanical clearing, land restoration is rarely required. Clearing land among the approximately 50 million ha of tropical swamps, mainly for rice production, is a special case. Most pilot projects have not lived up to expectations and the feasibility of clearing tropical swamps remains questionable. On the one hand, there are secondary problems such as acid sulphate capacity and, on the other hand, opposition from the general ecological movement towards the preservation of the remaining wetland ecosystems. One of our highest priorities must therefore be the restoration of land and the development of technologies better able to exploit fragile soil conditions. The International Board of Soil Research and Management (IBSRAM) is a young institution engaged in organizing networks to develop suitable management technologies, especially for problem soils (estimated to cover about 0.7 billion ha), and has made this a priority in order to promote soil restoration through its cooperative networks. Other organizations for development or technical cooperation have to make their own contributions to the restoration of degraded soils so that they can be returned to sustainable agricultural productinn Prof. Dr. H.W. Scharpenseel of the Universitv of Hamburg is a member of the German Council for Tropical and Sub-tropical Agricultural Research as well as the Hamburg State Council for the Environment.H.W. ScharpenseelLand restoration: a neglected taskThere is much talk of the ecological disasters which result from clearing land by slash and burn: lowering of the water table, spreading savannah colonized by Imperata cylindrica, and..

Calculating the annual input of organic matter to soil from radiocarbon measurements

Measurements of total organic C, deltaC-13 and deltaC-14 are given for topsoils taken from six experimental sites in southern England. At each site, some of the samples were collected before and some after the thermonuclear tests of the early 1960s, so that pre- and post-bomb samples could be compared for radiocarbon content. The current Rothamsted model for the turnover of organic C in soil gave an acceptable fit to the data from five of the sites, apart from one aberrant radiocarbon measurement. The annual input of C to the topsoil was calculated for the five sites from these fits; the values obtained were: 0.15 t C ha-1 a-1 for a site on silty clay loam, kept bare by hand weeding since 1870; 0.2 for unmanured spring barley growing on a sandy loam; 2.95 for a fertilized all-arable rotation on a loamy sand; 1.9 for the same fertilized all-arable rotation on a silty loam and 2.5 from this rotation on a calcareous silty loam. The corresponding values for Net Primary Production at the five sites were 0.15, 0.76, 5.16, 5.71 and 5.46 t C ha-1 a-1. In fitting the model to the radiocarbon data it was necessary to postulate that all these sites contained substantial quantities of biologically-inert organic matter, ranging from 2.2 to 10.0 t C ha-1

The turnover of organic carbon in subsoils. Part 1. Natural and bomb radiocarbon in soil profiles from the Rothamsted long-term field experiments

The Rothamsted long-term field experiments, started more than 150 years ago, provide unique material for the study of carbon turnover in subsoils. Total organic C, 14C and 13C were measured on soil profiles taken from these experiments, before and after the thermonuclear bomb tests of the mid-20th century. Four contrasting systems of land management were sampled: land cultivated every year for winter wheat; regenerating woodland on acid soil; regenerating woodland on calcareous soil; and old grassland. The mean radiocarbon ages of all the pre-bomb samples from cultivated land were 1210 years (0–23 cm), 2040 years (23–46 cm), 3610 years (46–69 cm) and 5520 years (69–92 cm). Bomb radiocarbon derived from thermonuclear tests was present throughout the profile in all the post-bomb samples, although below 23 cm the amounts were small and the pre- and post-bomb radiocarbon measurements were often not significantly different. Values of δ13C increased down the profile, from −26.3‰ (0–23 cm layer, mean of all measurements) to −25.2‰ for the 69–92 cm layer. The C/N ratios decreased with depth in virtually all of the profiles sampled. Excluding the surface (0–23 cm) soils from the old grassland, the hyperbola m = 152.1 − 2341/(1 + 0.264n) gave a close fit to the radiocarbon data from all depths, all sampling times and all sites, where n is the organic C content of the soil, in t ha−1, and m is the radiocarbon content of the soil, in Δ14C units, corrected for expansion or contraction of soil layers with time. The aberrant grassland soils almost certainly contained coal: one of them was shown by 13C-NMR to contain 0.82% coal C. In Part 2 (this issue) of this pair of papers, these radiocarbon and total C measurements are used to develop and test a new model for the turnover of organic C in subsoils