ABSTRACT

1. Land Reclamation Effects On Soil Nutrient Distribution in a Surface Mine Chronosequence in East Texas J.P. Ng, F.M. Hons, and T.J. Gentry. Texas A&M University, College Station, TX RESULTS ABSTRACT OTHER CHEMICAL PROPERTIES A problem in reclaiming surface mines to approximate native conditions is the poor soil quality that impedes revegetation in post-mined soils. Our objectives were to measure the distribution of chemical characteristics, including nutrients, in the soil profile to 1 m over a chronosequence of 40 years to determine when a reclaimed soil returned to premined conditions. In addition, we compared mine soils subjected to two different reclamation practices [crosspit spreader (CP) and mixed overburden (MO)] at 20 years of age at Big Brown lignite mine in East Texas. Soil quality indicators, including soil organic carbon, were able to reach and exceed premined concentrations almost immediately after reclamation began, although the distribution through the soil profile required at least 5 years before stratification was observed. Nitrogen maintained premined levels and profile distribution after 15 years of reclamation, while P was able to stratify after 5 years but not reach premined conditions. Other nutrients were able to exceed premined conditions and develop a similar native profile distribution in 10-15 years (K and Ca), while Mg, S, and Fe displayed an increase in depth. When comparing the two reclamation practices at 20 years of age, the CP showed greater stratification between 0-15 and soil deeper than 15cm, but lower concentrations of nutrients compared to the MO. The stratification of soil nutrients in post-mined soils indicated a return of biological activity, which is strongly influenced by the processes used to regrade the disturbed landscape and the timing of revegetation. Changes in soil textural classification appear to correlate with the increase of nutrients during site rehabilitation. We conclude that the more recent implementation of CP served as a better method to return mined soils to premined conditions, but more research should be conducted to determine how aggregates and biological activity from the soil microbial community are influencing the accumulation of soil organic carbon during surface mine reclamation. • Physical Properties • Texture varied greatly between sites, but uniform through depth. • Sandy in NP. • Sandy loam and sandy clay loam in CP sites • Clay loam in MO sites. • Axtell, Lufkin, Tabor series were predominant native series. • Bulk density in reclaimed soils through 15 years was higher than in unmined sites. • Bulk density increased with depth. • CP20 marked a return to NP levels. • All MO sites had lower bulk density than the NP. Fig 3. Operational dragline and crosspit spreader in an active surface mine. Fig 4. Reclaimed 20-yr old mixed overburden mine site (MO20). • pH • Decreasing pH was observed in CP sites with increasing age and depth except at CP0. • MO sites also had decreasing pH with depth, but were similar with age. • CP soils were more acidic than MO soils. • Stratification of pH was observed after 5 yrs. • Electrical Conductivity • EC in reclaimed sites increased with depth in comparison to the NP site. • EC in soils deeper than 15 cm was lower than topsoil after 15 yrs. • In CP sites, EC decreased after initial reclamation. • MO EC was consistent in all sites. • Cation Exchange Capacity • NP CEC was lower than CP reclamation sites • No apparent trend could be observed with age. • Stratification was not observed in CP sites. • Data is only available for NP and first 10 yrs of CP reclamation. • Lignite CEC = 111.1 meq/100g. Soil Carbon and Nitrogen Lignitic Carbon and Nitrogen • Lignitic Carbon • Carbon increased with age of reclamation. • CP lignitic C was uniform with depth • MO lignitic C was variable, and had no trend with depth, but had highest percentages of lignitic C. • More lignitic C was detected in MO v. CP. • Lignitic Nitrogen • Lignitic N was higher in reclaimed sites than NP. • There was an apparent trend of increasing nitrogen with age. • There was no strong trend of N with depth. • More N was detected in MO compared to CP. • Carbon • Soil C in NP displayed a decreasing trend with depth. • Reclamation sites had higher a C% in 0-15 cm, but similar C% at lower depths. • Variability was highest shortly after reclamation and ≥20 yrs. • C% increased with age. • Nitrogen • Soil N in NP displayed a decreasing trend with depth. • Not until CP15 does is this trend observed. • Nitrogen at ≥20 yrs increases compared to NP. • Carbon to Nitrogen Ratio • At 0-15 cm, ratios stay between 7.3 and 16.2 for all ages of reclamation. OBJECTIVES Determine how soil properties change over time by selecting sites of different ages that are presumed to have been treated the same. Determine the length of time required to achieve undisturbed (pre-mined) soil quality by focusing on multiple interrelated determinants of soil quality METHODOLOGY • Sites had a uniform progression of age, minimal slope, and on top of hills to avoid accumulation of runoff and nutrient variation. • Vegetation: Coastal bermudagrass, Yuchiarrowleaf clover, and Crimson clover. • Sampling occurred on June 23 and 24, 2009. • Five sites were selected with the crosspit spreader (CP) reclamation treatment spanning 0 to 20 years: CP0, CP5, CP10, CP15, and CP20 • Three sites were selected with the older mixed overburden (MO) treatment aged 20 to 40 years: MO20, MO30, and MO40 • One unmined site was selected as a control: NP CONCLUSIONS • Lignite contributed to both C and N levels, but only in reclaimed soils. The presence of resistant C and N in NP samples suggests a form of non-coal resistant organic material. • C/N ratios in 0-15 cm suggest a potentially active soil microbial community with net mineralization rates. • Nutrient stratification was observed after 10 years of reclamation. Greater stratification was observed in CP compared to MO sites. This was probably due to variability from the older reclamation technique (MO), longer-term plant growth, root penetration and nutrient uptake. • Nutrient concentrations were mostly higher in MO compared to CP sites. • pH decline over time is an area of concern as it may indicate the presence of acid-forming materials. However, it could also indicate an increase in soil respiration resulting in carbonic acid. • Although EC levels increased dramatically after reclamation, levels were lower than the threshold range considered detrimental to plant growth (> 1 mmhos/cm). Fig 1. Big Brown Mine is located near the town of Fairfield, TX. Phosphorus and Potassium Secondary Nutrients • Calcium • Increased dramatically after reclamation. • Stratification seemed more consistent ≥20 yrs. • MO > CP with the exception of Ca levels at CP0. • Magnesium • Increased dramatically after reclamation. • Mg levels appeared to increase with depth in all reclamation sites. • Sulfur • S levels were higher after reclamation compared to NP. • At 15 yrs of reclamation, S levels in 0-15 cm decreased closer to NP levels. • After reclamation, S increases with depth. A better trend with depth is observed at 10 years. • Phosphorus • P in the NP site was higher than all reclaimed sites. • Less stratification was observed ≥ 15 cm in reclaimed sites. • CP sites peaked at 15 yrs. • MO P levels were similar between 20 and 40 yrs. • At 20 yrs, MO had higher P than CP sites, but only in 0-15 cm. • Potassium • K in the NP site was lower than all reclaimed sites. • Stratification did not occur until 10 yrs after reclamation. • K levels were highest at 10 years in CP sites. • MO K levels were similar by age. • At 20 yrs, MO had higher K than CP sites. • Four transects 30-m long at each site. • 10 1-m cores were taken along each transect. • Each soil core was divided into 5 lengths: 0-5, 5-15, 15-45, 45-75, and 75-100 cm, and combined. • SOC, TC, and TN analyzed by the combustion procedure. • Chemi-thermal method was used to determine lignitic C and N content. • Mehlich III and a DPTA solution were used for all other nutrients. FUTURE RESEARCH • Taxonomic and functional microbiological data will be gathered and correlated with the changes in soil data. • Determine soil aggregation and the activity of each fraction, including measurements in soil respiration and soil microbial biomass. Fig 2. Nine sites selected for the chronosequence in Freestone County, TX. We acknowledge Luminant for providing research funding and support. Questions or Comments? Please contact me at justin.ng@tamu.edu