Stability of soil organic matter under long-term biosolids application

  • What is plow layer?
    • top 15-20 cm (control 15 cm, biosolids applied increase this by 15-20 cm); accounted for this by sampling up to 30 cm in amended fields
  • mined vs. unmined soils
    • loss of topsoil due to stripmining in the early 1900s (Peterson et al. 1979); even greater than 30-50% loss of org-C since cultivation and drainage of prairie
  • received municipal biosolids (wet and dry) from 1972 to 2004; sampled 2006; see details of characteristics in Tian et al. 2009
    • 1416 Mg ha-1 in mined Entisol soil (2x farmland application rate, FAR); N-loading 41.6; pH 7.7
    • 1072 Mg ha-1 in unmined Alfisol (1.6x FAR); N-loading 33.8; pH 6.7
    • reference field Mollisol given its highest and most stable SOM among arable soils in the region
    • fields used for corn, wheat, soybean and fallow in rotation
      • how did this negatively impact C and SOM?
  • biosolids application increased soil microbial biomass C by 5x in mined and 4x in nonmined soil
  • amended soils showed
    • high basal respiration and N mineralization
    • low metabolic quotient
      • the ratio of microbial respiration to biomass; used to evaluate ecosystem maturity; matured stable systems tend to have a low qCO2 (Anderson and Domsch (1990)); used to evaluate SOM stability under addition of C sources
      • low rate of organic C and org-N mineralization
  • increase in mineral-associated C from
    • mined soil: 6.9 (fertilizer control) to 26.6 g kg-1 (3.9x)
    • unmined soil: 8.9 to 23.1 g kg-1 (2.6x)
  • amorphous Fe and Al (improves SOM stability) increased 2-7x
    • Amorphous (oxalate extractable) iron and aluminum oxides are known to stabilize organic matter in soil (e.g. Kaiser and Guggenberger, 2000; Kleber et al., 2005; Schwertmann et al., 2005)
    • biosolids contain 2-5% Fe and 1-2% Al, mostly in amorphous form
  • Overall conclusion: biosolids amendment increases SOM stability

Questions:

  • after 32 years of biosolids application, improvements shown
    • how does this scale of improvement compare to others?
      • e.g. grazing studies (examples of pos and neg)
      • Marin Carbon Project
      • no-till and other soil conservation practices
  • Is there an emergy analysis of biosolids application, e.g. fossil fuels, labour, equipment needs, involved?
  • regarding heavy metals in biosolids: “Since the promulgation of the regulations, the heavy metal concentrations in biosolids have been largely reduced, and the use of this product in farmland has increased steadily, being estimated to be 50% in 2010 for the 8.0 million dry tons in US (USEPA, 1999).”
  • established that they increase SOM, but not known how biosolids impact SOM stability
  • The control fields received chemical fertilizer at annual rate of about 300 N, 100 P and 100 K in kg ha−1… did others at same rates? Can we assume yes given they were cropped?

Methods

  • The soil microbial biomass C (SMBC) was measured using the fumigation–incubation method. The soil sample was moistened and pre-incubated for 10 days. At 10 d, samples were removed from the incubation jar, and fumigated with CHCl3 under vacuum, and vapors removed at 24 h. The fumigated samples were placed in a separate canning jar along with vials of alkali and water for incubation at 25 ◦C for 10 d. The soil microbial biomass C was calculated as the quantity of CO2–C evolved during the incubation of the fumigated sample divided by an efficiency factor of 0.41 (Voroney and Paul, 1984; Franzluebbers et al., 1999). The metabolic quotient (qCO2) was calculated as the ratio of basal respiration to SMBC (Anderson and Domsch, 1990). The basal respiration was the CO2–C evolvement from 10 to 24 d of incubation (Franzluebbers et al., 2000). Similarly, the qNMIN was calculated as the ratio of N mineralization to SMBC.
  • For the determination of soil amorphous Fe and Al, the soil samples were extracted by 0.2M acid ammonium oxalate (Leoppert and Inskeep, 1996). Then, the soil extract was digested in the presence of concentrated HNO3 to dryness, and the residue was redissolved by 1MHCl. The Fe and Al in the solution were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

Statistics

  • calculated mean values for two depths (0-15 cm and 15-30 cm for treatments) for amended soils and compared to 0-15 cm in control
    • in mined soil especially, then, how much are they simply averaging in the biosolids material?
  • used a simple paired t-test
  • this makes a lot of assumptions about normality and lack of confounding variables…

Conclusions & Discussion

  • The slower mineralization rates of organic C and N in the biosolids-amended soils suggest the biosolids application leads to the increased stability of SOM in the arable soil. Before biosolids were applied to fields, they underwent a series of stabilization processes such as the anaerobic digestion, lagooning and/or air-drying, which could lead to the accumulation of recalcitrant organic matter.
    • How is this accounted for in an overall carbon-neutrality accounting?  e.g. emergy analysis
  • large importance of amorphous Fe and Al oxides in stabilization of SOM
  • increase C and N mineralization… but from the Discussion: “The increase in soil C mineralization under a certain management could increase the plant available nutrients, leading to greater plant biomass production, as Janzen (2006) pointed out that soil organic matter management involves finding ways to build and use the C. On the other hand, the greater soil C mineralization can lead to increased
    soil C loss.” Does this imply that improved/increased nutrient cycling with living plants should be used to prevent soil C loss?

Further Reading

Anderson, T-H., Domsch, K.H., 1990. Application of eco-physiological quotients (qCO2, and qD) on microbial biomasses from soils of different cropping histories. Soil Biol. Biochem. 22, 251–255.

Drinkwater, L.E., Workneh, F., Letourneau, D.K., van Bruggen, A.H.C., Shennan, C. Fundamental differences in organic and conventional tomato agroecosystems in California. Ecol. Appl. 5, 1098–1112.

Franzluebbers, A.J., Langdale, G.W., Schomberg, H.H., 1999. Soil carbon, nitrogen, and aggregation in response to type and frequency of tillage. Soil Sci. Soc. Am. J. 63, 349–355.

Gunapala, N., Scow, K.M., 1998. Dynamics of soil microbial biomass and activity in conventional and organic farming systems. Soil Biol. Biochem. 30, 805–816. Link

Janzen, H.H., 2006. The soil carbon dilemma: shall we hoard it or use it? Soil Biol. Biochem. 38, 419–424.

Six, J., Conant, R.T., Paul, E.A., Paustian, K., 2002. Stabilization mechanisms of soil organic matter: implications for C-saturation of soils. Plant Soil 241, 155–176.

Tian, G., Granato, T.C., Cox, A.E., Pietz, R.I., Carlson Jr., C.R., Abedin, Z., 2009. Soil carbon sequestration resulting from long-term application of biosolids for land reclamation. J. Environ. Qual. 38, 61–74.

Wallace, B.M., Krzic, M., Forge, T.A., Broersma, K., Newman, R.F., 2009. Biosolids increase soil aggregation and protection of soil carbon five years after application on a crested wheatgrass pasture. J. Environ. Qual. 38, 291–298.

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