Crop residue management received little attention until about 1970. Records of crop residue production are limited, but crop yield databases have been available since 1865. Carbon sequestration and other conservation benefits require a detailed knowledge of crop residue production and management. Our objectives are to: (i) review grain and biomass yield, harvest index (HI), and root C/shoot C ratios (k) of major grain crops in the USA; (ii) discuss historical agriculturalpractice impacts on soil organic C (SOC); and (iii) compare estimates of total (above-and belowground) source C production (ESC) relative to minimum source C inputs required to maintain SOC (MSC). Aboveground MSC input averaged 2.5 6 1.0 Mg C ha 21 yr 21 (n 5 13) based on moldboard plow sites and 1.8 6 0.44 Mg C ha 21 yr 21 (n 5 5) based on no-till and chisel plow sites. These MSC values included only aboveground source C, thus underestimate the total MSC. When ESC is estimated from k, including rhizodeposition (k rec ), the true magnitude of the C cycle is at least twice that when ESC is estimated using k excluding rhizodeposition (k his ). Neglecting rhizodeposition C underestimates the net production of C in cropland. Current yields and measured MSC predict continued SOC loss associated with soybean [Glycine max (L.) Merr.] and some wheat (Triticum aestivum L.) production management unless conservation tillage is used and ESC is increased. The adequacies of ESC to maintain SOC has direct implications for estimating the amount of crop residue that can be harvested and yet maintain SOC.

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Seven crop residue treatments were initiated in 1931 to measure long‐term residue management effects on soil organic matter in a wheat‐fallow cropping system on Pacific Northwest semiarid soils. There was evidence at this time of substantial organic matter (OM) loss during the first 50 years of wheat cultivation in the Great Plains. Organic carbon (C) and total (N) were measured at approximately 11‐year intervals over a 45‐year period to determine residue effects on the rate of change in soil OM content.Only the addition of 22.4 metric tons of manure/ha to straw residue before incorporation prevented a decline in soil N and C. The addition of 45 or 90 kg fertilizer N or of 2.2 metric tons of pea vines/ha to straw residue before incorporation reduced N and C loss when compared to straw only incorporation. Burning of straw in the fall following wheat harvest accelerated the loss of N but not C. Burning of straw in the spring just prior to tillage had no effect on N or C loss.Changes in N and C were primarily confined to the top 20 cm of soil. Soil C/N ratios in 1976 differed between treatments proportional to the rate of N loss; they were highest in burn or straw only treatments and lowest in the manure treatment.In all treatments, changes in soil N were best described by a linear function of time; slope within the linear function depended upon residue treatment. This linear function of time over a 45‐year period following approximately 50 years of previous cultivation suggests that 100 or more years may be required before N levels become stationary. Residual effects confirm that the new stationary level will depend on past crop residue management practices.Changes in soil C correlated highly with the amount of organic C supplied by each treatment, regardless of the different kinds of residue applied. Thus, changes in soil organic matter levels were controlled primarily by the amount of organic C supplied in crop residue. Regression equations indicate that approximately 5 metric tons of mature crop residue ha−1 year−1 are needed to maintain soil organic matter at its present level when cropped in wheat‐fallow rotation in this climatic zone.

It is often necessary to measure soil water content at multiple points in space and time. Our goal was to develop an automated and multiplexed measurement system using time‐domain reflectometry (TDR). Two systems are described; the first (A) uses an analog TDR unit, in which voltage signals sent from the TDR to a datalogger convey the shape of the waveform. The second (D) uses a digital TDR that communicates a digital representation of the oscilloscope wave to a personal computer. Both systems use the same multiplexing strategy, in which the TDR transmission line connects through a 12‐position rotary switch to various waveguides positioned in the soil and to further rotary switches. The switches are turned by stepping solenoids that are activated by the datalogger in System A and the computer in System D. System D uses software to automatically analyze the incoming waveforms and calculate volumetric water content. Some of the possible uses of each system include observation of infiltration at multiple points within a field and measurement of unfrozen water content as a function of space and time during freezing and thawing. The system has also been used to estimate the reproducibility of water content measurement by TDR, which was found to be in the range of ±0.006 to ±0.008. The systems described should be useful for field research on many subjects, including studies of transport and biological processes in soil, and validation of root water‐uptake models.

Recent studies of SOC storage and turnover have employed 13 C natural abundance (␦ 13 C) as an in situ Soil organic carbon (SOC) is sensitive to management of tillage, marker of relic and recent SOC pools. Mass concentraresidue (stover) harvest, and N fertilization in corn (Zea mays L.), tions of SOC and the ␦ 13 C signature are sufficient to but little is known about associated root biomass including rhizodeposition. Natural C isotope abundance (␦ 13 C) and total C content, mea-calculate the amount of SOC originating from a C 4 crop sured in paired plots of stover harvest and return were used to estimate (e.g., corn) or from a C 3 crop [e.g., soybean, Glycine corn-derived SOC (cdSOC) and the contribution of nonharvestable max (L.) Merr.] when the initial soil organic carbon biomass (crown, roots, and rhizodeposits) to the SOC pool. Rhizo-(SOC i) has a different 13 C signature than the current deposition was estimated for each treatment in a factorial of three crop (Balesdent et al., 1987). The ␦ 13 C technique has tillage treatments (moldboard, MB; chisel, CH; and no-till, NT), two shown that tillage influences the depth distribution of N fertilizer rates (200 and 0 kg N ha Ϫ1), and two corn residue manage

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Conservation production systems combine tillage and planting practices to reduce soil erosion and loss of water from farmland. Successful conservation tillage practices depend on the ability of farm managers to integrate sound crop production practices with effective pest management systems. More scientific information is needed to determine the relations between tillage practices and physical, chemical, and biological soil factors that affect plant and pest ecology. There is a need to devise improved pest management strategies for conservation tillage and to better understand the impact of conservation tillage on water-quality, especially as it is related to use of agricultural chemicals. While savings in fuel, labor, and soil have induced many farmers to adopt conservation tillage, improved methods and equipment should increase adoption even more.