I am developing a phosphorus (P) budget for an agricultural watershed. Chemical fertilizers and organic matters (compost and manure) are the main sources of P in my study site. In addition, the soil of the watershed is characterized as sandy loam. P sorption capacity of manured sandy loam soils is lower than chemical fertilized soils. So, can I conclude like that P coming from organic matters are mainly exported through waterways. Or is there any way to identify which part of P mainly exported from the watershed hydrologically?
Dear Reza,
Please chech the following paper out to see equations to determine relations between organic and inorganic P in sediments.
Regards
Article Sediment Quality in Arkansas Bait Minnow Ponds
from my experience, your system is transports organic P (either adsorbed on suspended particulate not from the sediment but the remaining of fertilizers or dissolved). you can check my thesis attached phosphorus and nitrogen budget in Al-Hawizeh marshland, IRAQ"
The question is a good one and as a chemist I am not happy with any of the responses so far. Most research relies on measuring molybdate reactive P before/after treatment of soil solutions/ runoff/ flow-through to discriminate between organic and inorganic forms of P in solution. Unless the species are first separated molybdate reactivity is an indiscriminate tool. And separation itself is complex because much of the P, even if present as phosphate, may in fact be part of suspended colloids. I'm sorry to say that separation followed by measurement is the way to go, but I'm unaware of any authoritative research on the topic, i.e. selective measurement is probably a project in itself. Were I to go there I would use chromatographic separation with dual post column detection, e.g. using ICP on one leg and molybdate on the other. On the ICP leg I would simultaneously measure Fe, Al and C for starters as indices of the forms in which the P may be present. Good luck!!
Your question actually relates to the "disconnect" between what is measured (Total P and soluble reactive P, SRP) in stream monitoring and the phosphorus forms (Organic P and inorganic P) of watershed models like SWAT. While not exactly the same, modelers assume dissolved P as the inorganic P and particulate P (total P -SRP) as the organic P. Most stream monitoring measures total P and SRP only (See my ASABE conference paper "Modeling Dissolved Phosphorus Exports in Lake Erie Watersheds"). It's very rare that you see organic P and inorganic P in reports. We have a very good discussion about phophorus forms in this paper: Phosphorus loading to Lake Erie from the Maumee, Sandusky and Cuyahoga rivers: The importance of bioavailability, Journal of Great Lakes Research.
Some thoughts, avoid using organic and inorganic. Try total P, and non-molybdate reactive P and molybdate reactive P
also consider the application of organic sourced P can displace inorganic P as if what your dealing with is not complicated enough. Also, there is a fair amount of inorganic P in manure...
To give a good answer I think i would need to know much more about the specifics of the research or proposal.
Michael is correct in saying that much of the P in manure is inorganic, as indeed it can be in plant residues
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