1. Effect of Organic Farming on Labile Carbon Fractions in Soils of Southern New Mexico
Y. Ikemura1, P. A. Jacinthe3 and M. K. Shukla2
1Environmental Services, Pacific Golf Management, Tokyo, Japan
2Department of Plant and Environmental Sciences, New Mexico State University, Las Cruces, NM
3Department of Earth Sciences, Indiana University- Purdue University, Indianapolis, IN
Introduction
Soil organic matter (SOM) is essential to maintain productivity of organic farming systems. These systems rely on the application of manure to sustain the pool of nutrients available for plant growth. The practice can potentially contribute to soil C sequestration and improved soil quality. However, since the application of agro-chemicals is not permitted in organic farming, deep tillage becomes necessary for the control of weeds in organic farms. In addition, state regulations also make tillage mandatory after the harvest of some crops (example: cotton). Therefore, increased frequency of tillage operations can exacerbate SOM mineralization resulting in diminished rate of SOM accretion, specially in arid region soils.
Because the bulk of SOM consists of carbonaceous fractions with long turn over times, total SOM content of soil alone may not accurately reflect small changes in soil fertility status, especially during the first years of organic farming. In these agro-ecosystems, biologically-active SOM fractions, and SOM aromaticity indexes (H:C, O:C ratios) may be more sensitive indicators of SOM quality and nutrient cycling than total SOM.
This study was conducted in in a chronosequence of conventional and organic farms in the semi-arid ecosystem of southern New Mexico with the objectives of:
determining the impact of organic farming duration on soil carbon accumulation, and
assessing the effect of organic farming on labile C pools.
Soil Sampling and Analysis
Bulk soil samples were collected in triplicate at 0-10, 10-20, and cm depths in September 2006 (Fig. 1). Soil samples were air-dried, and finely-ground (250 μm).
Determination of total C, N and H by dry combustion (Thermo Electron CHNS-O analyzer),
Determination of inorganic C via decomposition of carbonates by mild acid (2 M HCl) and measurement of evolved CO2 using gas chromatography (Varian CP 3800).
Organic C was computed as the difference between total and inorganic C.
Results
Total and characteristics of soil organic matter in conventional farm (CON), and after 3 (OF3), 6 (OF6), and 9 (OF9) years of organic farming. Bar denotes standard deviation.
Fig. 6. Microbial biomass carbon (mg kg-1 soil) obtained by the substrate-induced respiration method.
Fig. 5. Soil respiration (mg CO2-C/kg soil/d) determined as the average rate of CO2 production during a 20-d incubation.
Fig. 4. Hydrogen-Carbon ratio of soil organic matter
Fig. 2. Soil organic carbon (%).
Fig. 3. Carbon-Nitrogen ratio of soil organic matter.
Materials and Methods
Four farms were selected for the study:
- Three organic farms: 3 (OF3), 6 (OF6), and 9 years (OF9) of organic farming.
- One conventional farm.
All farms are located near the town of Anthony in Dona Ana County, New Mexico (Fig. 1). At all sites, surface soil texture is predominantly loam-silt loam. Climate is arid with mean annual temperature of 20oC. Mean annual precipitation between 180 and 230 mm mostly during May and August. Fields are flood-irrigated when planted to alfalfa; for all other crops, farms are irrigated by the furrow irrigation method.
Organic farms:
a) Rotation: predominantly alfalfa/cotton (Gossypium arboreum L) with occasional chile (Capsicum annuum) and lettuce (Lectuca sativa).
b) Manure: 0.18 Mg ha-1 of dry chicken pellets in alfalfa years; 50 Mg ha-1 of dry cow manure in all other crops.
c) Tillage: annual mold board plowing (35 cm deep) followed by chisel tillage.
Conventional farm:
a) Rotation: Planted to cotton for the past six years; rotation of cotton and corn earlier
b) Fertilization: 113 L ha-1 of (N-P-K) at pre-plant, followed by two applications (113 L ha-1 each) of urea ammonium nitrate solution (URAN; N-P-K).
Summary
Although SOC content was generally similar (Fig. 2), significant difference in SOM quality can be noted among farming practices.
At most soil depths, microbial biomass carbon was significantly higher in the organic farms than in the conventional farm soil, but trend with respect to the duration of organic farming was less consistent (Fig. 6).
Soil respiration was generally higher in the conventional than in the organic farm soil samples (Fig. 5). It follows that the metabolic quotient (Fig. 8), was 3-6 fold higher in the conventional farm soils.
The molar HC ratio of soil organic matter increased with the length of time under organic farming suggesting a decrease in SOM aromaticity. This interpretation is consistent with the observed positive relationship between soil respiration and HC ratios (Fig. 7).
Fig. 8. Metabolic quotient (mg CO2-C mg-1 microbial biomass C.
Fig. 7. HC ratio and soil respiration relationship.
Fig. 1. Location of experimental farms and sampling sites near the town of Anthony, NM.
Acknowledgment: This research was supported by the Agriculture Research Station of New Mexico State University. The authors acknowledge the help and support of organic grower Mr. Ramon Alvarez. Laboratory assistance of Alice Enochs, Jonathan Bills and April Herman is also acknowledged.