No abstract

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Loss and Recovery of Soil Organic Carbon andNitrogen in a Semiarid Agroecosystem Soil Fertility & Plant Nutrition S tudies of the eff ects of cultivation on SOC in the western United States show losses of 20 to 40%, much of which occurs during the fi rst 10 yr aft er tillage began (reviewed by Davidson and Ackerman, 1993). Th ough these impacts represent drastic soil degradation, they may underestimate eff ects of early, intensive cultivation that included few inputs for soil fertility renewal. By the time studies of cultivation eff ects were conducted, farming practices had changed markedly from methods used during the fi rst 50 to 70 yr aft er the prairie sod was broken

Low inherent soil fertility in the highly weathered and leached soils largely accounts for low and unsustained crop yields in most African countries. But in particular, the major nutrients, nitrogen (N) and phosphorus (P), are commonly deficient in these soils. This scenario of nutrient depletion is reflected in food deficits and hence the food aid received continuously, specifically in sub-Saharan Africa. Undoubtedly, substantial efforts have been made in the continent to replenish the fertility of degraded soils in attempts to raise crop yields, towards self-sufficiency and export. Such efforts consist of applications of both organic and inorganic resources to improve the nutrient status of soils and enhanced nutrient uptake by crops, provided that soil moisture is adequate. Overall, positive crop responses to these materials have been obtained. Thus in the East African region, maize (staple) yields have been raised in one growing season from below 0.5 t/ha without nutrient inputs, to 3-5 t/ha from various nutrient amendments at the smallhold farm level. However, in spite of the positive crop responses to nutrient inputs, farmers are generally slow to adopt the soil fertility management technologies. In this paper we review the impact of some technologies, focussing the use of nutrient resources of different characteristics (qualities) in relation to improved crop yields, with an overall goal to enhance technology adoption. Thus, inorganic resources or fertilizers often give immediate crop responses, but their use or adoption is rather restricted to large-scale farmers who can afford to buy these materials. Organic resources, which include crop residues, water hyacinth and agroforestry shrubs and trees, are widely distributed, but they are generally of low quality, reflecting the need to apply large quantities to meet crop nutrient demands. Moreover, most organics will add N mainly to soils. On the other hand, phosphate rocks of varying reactivity are found widely in Africa and are refined elsewhere to supply soluble P sources. The recently developed soil fertility management options in East Africa have targeted the efficient use of N and P by crops and the integrated nutrient management approach. Some people have also felt that the repackaging of inputs in small, affordable quantities, such as the PREP-PAC described in this paper, may be an avenue to attract smallhold farmers to use nutrient inputs. Nonetheless, crop responses to nutrient inputs vary widely within and across agroecozones (AEZs), suggesting specificity in recommendations. We highlight this observation in a case study whereby eight soil fertility management options, developed independently, are being tested side-

No abstract

Soil organic matter (SOM) fractions were determined using extraction‐, incubation‐, and density‐based fractionation techniques on samples collected from a range of furrow‐irrigated sugar beet (Beta vulgaris L.) based rotations on the same soil series on farmers' fields in Wyoming. We hypothesized that extending the period of time between sugar beet crops in rotations beyond the 2‐yr sugar beet–barley (Hordeum vulgare L.) (SB‐BA) rotation by adding perennial or annual legumes would lead to higher levels of surface‐soil (0–15‐cm) organic C and N. Four rotations were compared: SB‐BA, sugar beet–dry bean (Phaseolus vulgaris L.) (SB‐DB), sugar beet–barley–dry bean (SB‐BA‐DB), and sugar beet–sugar beet–alfalfa (Medicago sativa L.)–alfalfa (SB‐SB‐Alf‐Alf). Soils under SB‐BA and SB‐DB rotations on average contained 607 g soil organic C (SOC) m−2 in the upper 15 cm, or 46% of the SOC found within SB‐BA‐DB and SB‐SB‐Alf‐Alf soils. Potentially mineralizable C and N and microbial biomass C (MBC) were lower in SB‐BA and SB‐DB soils than SB‐BA‐DB and SB‐SB‐Alf‐Alf soils, but, when normalized by SOC and total soil N (TSN), these labile C and N fractions were >1.5 times higher in SB‐BA and SB‐DB soils, suggesting greater SOM mineralization. Moreover, light‐fraction C in SB‐BA and SB‐DB soils was about half that of SB‐SB‐Alf‐Alf soils. Sugar beet sucrose yield was also higher in the SB‐SB‐Alf‐Alf than any other rotation. There were strong linear relationships (r2 = 0.50–0.84) between sugar beet sucrose yield and TSN, SOC, and MBC across all four rotations. To conserve high surface‐soil organic C and N fractions on furrow‐irrigated farm fields without sacrificing sugar beet sucrose yield, extending the 2‐yr SB‐BA rotation by adding 2 yr of alfalfa is recommended.