Soil Quality Changes In Different Residue Management Systems Compared To Grassland After 22 Years
Chemical analysis of surface soils of a Fargo silty clay for properties like organic matter (OM), pH, K, and Zn comparing Plow, Sweep and Notill tillage systems with an adjacent Grass area indicated OM and Zn levels decreased after 22 years, the greatest reduction with the Plow system. Soil pH increased in the Plow while K levels were relatively similar among tillage systems and grass. Stratification of nutrients occurred where more residue was left on the surface (Notill and Grass). Both carbon and nitrogen returned to the soil was higher with tillage system than Grass. Carbon dioxide evolution from the soil varied with soil temperature and moisture with rates evolved greater with the Plow and Grass systems.
The objectives of this study are to: (1) measure the key chemical, physical and biological properties on a grassland area and on three long term 22-year residue management systems in a small grain-row crop rotation to determine the impact on soil quality; (2) determine carbon sequestration in the soil profile under the different systems; and (3) measure the evolution of carbon dioxide from the soil with the different systems under different N fertilizer variables over two different crop seasons
Objective 1 : Collection of random triplicate in-tack soil cores with plastic sleeves from three-replicated residue management systems (Plow, Chisel, Notill) and Grass areas was completed. Individual cores were separated into nine increments (0-3, 3-6, 6-12, 12-18, 18-24, 24-30, 30-36, 36-42 and 42-48 inches). Bulk density was determined on a segment of each increment using standard procedures. The remaining portion of soil from each segment was air dried and ground. Each soil sample is being analyzed for various chemical properties (OM, P, K, pH, EC, Zn, Fe, Mn, Cu, Ca, Mg, and Na) with some analyses yet to be completed. Replicate soil cores (8-in diameter by 6-in deep) were extracted from the grass and residue management systems. The soil in each core was hand sorted and washed over multiple sieves to determine the presence and number of earthworms and cocoons. Earthworm specie identification and separation into adults and juveniles still needs to be completed.
Duplicate soil samples were collected with a flat shovel from the surface 0-2 inch depth of each residue system or grass area and carefully placed in a flat cardboard box, returned to the lab and allowed to air dry. These samples will be evaluated for dry aggregate size distribution using a rotary sieve. Soil samples collected in the 0 to 3 and 3 to 6-inch depths with a small hand core sampling were returned to the lab and are currently being analyzed for microbial (bacteria and fungi) populations. All data collected will be summarized and evaluated statistically.
Objective 2 : A sub-sample of ground soil for each depth increment from the soil cores collected in Objective 1 was retrieved and finely ground with a ball mill. Analysis for carbon (organic and inorganic) and total nitrogen is currently underway. Upon completion of the analysis, the data will be summarized and statistically evaluated to determine differences in carbon, nitrogen and C:N ratios among systems and the location and amount of carbon sequestration in the soil profile. A durum wheat crop was planted the first year of the study with the amount of grain and residue determined at harvest. The amount of N in the grain and residue was determined. Replicate samples of dry matter production in the grass area were collected each year. These data allow for an estimation of the carbon and nitrogen removal and/or addition to the systems. Field pea was planted on the cropped area in the second year with different N fertilizer rates broadcast in the spring prior to planting. Results for pea crop are not completed at this time. Upon completion of all remaining data to be collected, the results will be summarized and statistically evaluated.
Objective 3 : A portable instrument (EGM or environmental gas monitor) that measures soil respiration was used to collect the amount of carbon dioxide released from the three residue management systems (0 and 120 lb N/acre fertilizer) and grass area at various times during the growing season. Only a portion of this large database has been summarized. Results are in the process of being statistically evaluated.
Impacts and Contributions/Outcomes
Some preliminary results from the first year of this project designed to measure soil quality changes in three different residue management systems compared to an adjacent grass area are presented below. Chemical analysis of soil profile samples indicates that the organic matter content (Fig. 1) in the surface 0 to 6-inch depth of the grass area was decreased with all three tillage systems. The conventional Plow system decreased almost 2% in the 0 to 3-inch depth and over 0.5% in the second depth. Continued loss of organic matter with this system will definitely impact future chemical, physical and biological properties of the soils at this depth. The two reduced tillage systems, Chisel and Notill, regained some of the organic matter loss, over 0.5% with NOTILL after 22-years. This is quite significant since changes in soils already high in organic matter are much slower and the relatively low residue (carbon) input with a small grain-grain legume rotation during the last 16 years of the study. Organic matter levels changed little in the 12 to 18-inch depth among the systems. Differences among systems are observed in the lower depth increments (24 to 36-inch) which suggests that movement of carbon may be associated with tillage or grass system.
Soil pH (Fig. 2) measured in the 0 to 12-inch depth was much higher with the Plow system indicating that extensive and/or deeper tillage depth have moved sub-soil bases cations closer to the surface. Continued increases, approaching 8.0 will definitely affect nutrient availability, reactivity of soil applied pesticides and biological activity. Soil pH below 18-inches was much lower on the cultivated area compared to the grass area. This suggests that cultivation may have modified the physical conditions of the soil with movement of water, up or down, along with base cations.
Potassium levels in the soil 0 to 6-inch depth profile (Fig. 3) were much higher in the Grass, Notill and Chisel systems and much lower in the 6 to 12-inch depth compared to the Plow system. This is fairly consistent with stratification of potassium near the surface associated with less tillage. Normally stratification in soils with high levels of potassium would not be considered a problem depending on soil water, crop grown and season. However, if potassium levels are further depleted from the lower profile, inadequate potassium levels in the soil and limited plant uptake may reduce crop yields when soil water is limited in the stratification zone during a dry season. Plants could conceivably respond to band applications of potassium fertilizers in the depleted zone with tillage systems like Notill. The depleted potassium zone in the Grass system (18 to 30-inch depth) suggests that the extensive undisturbed perennial grass root system depletes the subsoil of potassium faster than systems where crops are cultivated with annual crops where roots die at the end of the growing season.
Zinc levels in the soil (Fig. 4), which generally show a positive correlation with organic matter levels, were highest in the surface 0 to 6-inches and decreased with depth to 24-inches, irrespective of system. Removal of zinc with the seed of cultivated crop (small grain and legume) depleted zinc levels by 0.6 PPM with Notill to 0.75 PPM on the Plow system when compared to the Grass area. The slightly over 0.5 PPM level of zinc on the Plow system appeared to be near the critical stage where zinc fertilizer may be required for maximum production levels since zinc deficiency symptoms have been previously observed on grain legumes with this Plow system. However, No zinc deficiency symptoms were observed on the Notill system, which measured around 0.66 PPM.
Total, grain and residue dry matter production of durum wheat in one year (Table 1) were similar among the Plow, Chisel and Notill systems were similar and averaged around 7300, 2180 and 5130 LB/ac, respectively. Grass total dry matter averaged only 1750 LB/ac. The amount of carbon returned to the system, based on residue produced and residue at 42% carbon) averaged around 2150 LB/ac for in the small grain portion of the rotation and only 730 LB/ac for the grass system. The amount of carbon returned during the grain legume (data not yet available for this study) would probably be less since grain legumes produce less residue than small grain. Nitrogen concentration of the residue (Table 2) averaged around 0.79% with approximately 41 LB N/ac returned to the soil. This residue has a carbon:nitrogen ratio of 52:1. The grass material contains 0.90 nitrogen with 16 LB N/ac returned to the soil at a C:N ratio of 47:1.
Residue cover, predominantly durum straw (Table 2) after planting field peas averaged around 4, 39 and 76% for the Plow, Chisel and Notill, respectively. Grass cover remained at 100%. The cover, which also reflects the degree of tillage, was indicative of the total earthworms measured in the field (Table 2). Around 900 thousand earthworms per acre were found with the Plow system and increased another 190 thousand with the Chisel, which received only one tillage operation. Nearly 2 million earthworms were found under Notill. This reflects the impact tillage can have on reducing earthworm populations, especially if the earthworms are active when tillage is preformed. A population of around 800 thousand was measured in the grass area. We believe the higher populations on the cultivated area compared to the Grass is associated with differences in soil water and/or a legume in the rotation which is one management practice that can be employed to increase earthworm population.
The evolution of carbon dioxide (Table 3), which is a measure of the amount of carbon removed from the soil system, covered a large range among systems over time (range 0.17 to 1.61 grams/m2/hr). Highest CO2 evolution occurred in the spring months and decreased as the season progressed that coincided with soil temperatures. Evolution of CO2 usually decreased as soil moisture decreased. Plow and Grass systems generally had the highest release of carbon with the Chisel and Notill similar, but much lower than the Plow or Grass. The variation in gas evolution tended to be greater, although not always, where tillage was performed as compared to the more stable systems, Notill and Grass.
Note: If the reader would like to see a copy of the tables and figures listed in this report, contact the coordinator listed above by email or log onto the coordinator listed web site then click on Directory, Deibert, Recent Publications, and go through list of Reports for 2001 SARE Annual Report). The tables and figures can be downloaded as a WORD or PDF file.