Soil Organic Carbon Changes in Diversified Rotations of the Western Corn Belt

Sequestration and storage of carbon (C) by agricultural soils has been cited as one potential part of the solution to soil degradation and global climate change. However, C sequestration in soils is a slow and dynamic process. The objective of this study was to evaluate the effects of crop rotation and N fertilizer management on soil organic C (SOC) levels at several points in time during 18 yr of a long-term study in the Western Corn Belt. Seven cropping systems (three monoculture, two 2-yr, and two 4-yr rotations) with three levels of N fertilizer were compared. Soil samples were taken in the spring in 1984, 1992, 1998, and 2002 to a depth of 30 cm in 0to 7.5-, 7.5to 15-, and 15to 30-cm increments. No differences were obtained in SOC levels in 1984 at the beginning of the study. After 8 yr, rotation significantly increased SOC 449 kg ha across all cropping systems. From 1992 to 2002, SOC levels in the 0to 7.5-cm depth decreased by 516 kg ha across all cropping systems. Soil organic C levels in the 7.5to 15-cm depths in 1992 and 2002 demonstrated similar rotation effects to those in the surface 0to 7.5-cm, being not significantly affected from 1984 to 1992 but being significantly decreased from 1992 to 2002 (568 kg SOC ha across all cropping systems). Many of the SOC gains in the surface 30 cm measured during the first 8 yr of the study were lost during the next 10 yr in all but the 4-yr cropping systems after 18 yr. The loss of SOC in this latter period occurred when depth of tillage was increased by using a tandem disk with larger-diameter disks. These results demonstrate that more than one point-in-time measurement from long-term experiments is necessary to monitor SOC changes when several management variables, such as cropping system and N fertilizer, are being used. They also indicate that apparent small changes in cultural practices, such as in depth of tillage in this experiment, can significantly change SOC dynamics in the soil. Subtle changes in cultural practices (e.g., tillage depth) can have significant long-term results, but longterm experiments are required to quantify their impact under variable climatic conditions. INTEREST in C sequestration in soils and its potential for reducing emissions of greenhouse gasses has increased greatly in the last 10 to 15 yr. This interest is evidenced by the large number of studies into this topic. Jarecki and Lal (2003), through the review of 220 references, point out the need for adoption of what they call ‘‘recommended management practices’’ that will help fill the large C sink in the world’s agricultural soils. Such management strategies include crop rotations and tillage practices, cover crops, ley farming, agroforestry, use of organic waste materials, N fertilization, precision farming, and irrigation. West and Post (2002) presented a global analysis of SOC sequestration rates as affected by tillage and crop rotation from 67 long-term agricultural experiments from around the world. They indicated that increased C sequestration is obtained by decreasing soil disturbance, such as changing from conventional to no-tillage systems, or by enhancing rotation complexity. Enhancing of rotation complexity is defined as (i) a change frommonoculture to continuous rotation cropping, (ii) a change from crop-fallow systems to continuous monoculture or rotation cropping, and (iii) an increase in the number of crops used in a rotation cropping system. They noted that soil C sequestration rates in response to either of these types of management practice changes can have a delayed response, reaching peak sequestration rates between 5 and 10 yr after initiation and then declining to near zero after 15 to 20 yr. Similar conclusions have been reported in a review by Lal et al. (1998) based on results from Franzluebbers and Arshad (1996). It is important to examine these practices in long-term studies because of the amount of time required for many of these management practices to significantly change SOC content. Recently, several researchers have presented results from studies after 23 to 48 yr of different management scenarios. Allmaras et al. (2004) and Wilts et al. (2004) present information from a 29-yr study in Minnesota on the effects of harvest management on corn residue transformations and belowground materials into soil C. They reported that the aboveground stover contribution to SOC was greatest in the moldboard plow (MB) system followed by the chisel plow (CH) system and no-till (NT) systems, whereas below-ground (roots, crowns, exudates) contributions to SOC were greatest in NT systems followed by CH systems MB systems. Although these changes were sometimes small, the authors noted that these differences were obtained with small but subtle changes in management practices. Similarly, Booker et al. (2005) reported similar results on the long-term effects of tillage and corn stalk return on soil C dynamics from a study conducted for 28 yr in Connecticut. In Iowa, Russell et al. (2005) reported results taken from two studies 23 and 48 yr old on the effects of N fertilization and cropping system onC sequestration. In each of these reports, the importance of data from long-term studies becomes evident by quantifying the impact that small changes in management practices have with time. These reports present current SOC conditions after 23 to 48 yr of differentmanagement practices, but they provide little if any information on soil C dynamics throughout the duration of the studies. Most of the previous reports present measurements from one or two points in time, usually at the beginning and end of the study period. Many of them report differences in organic matter or some form of SOC at the current point in time because few, if any, of the studies were extensively sampled at their initiation. This G.E. Varvel, USDA-ARS and Dep. of Agronomy and Horticulture, Univ. of Nebraska, Lincoln, NE 68583. Joint contribution of USDAARS and the Nebr. Agric. Res. Div., Journal Series No. 14997. Received 31 Mar. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:426–433 (2006). Soil & Water Management & Conservation doi:10.2136/sssaj2005.0100 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 426 Published online February 2, 2006