Understanding how nitrogen moves with surface runoff: particulate and dissolved forms

Nitrogen in runoff isn’t a single thing. It’s a duo, a tag-team of forms that water picks up as it travels across fields and roads toward streams, rivers, and—ultimately—the Chesapeake Bay. If you’re studying Maryland nutrient management topics, you’ve probably run into this idea before: nitrogen moves in surface runoff in two main forms, and both can shape water quality in different ways. Let me walk you through what that means in plain terms, with a few Maryland-specific twists to keep it grounded.

PARTICULATE AND DISSOLVED: TWO WAYS NITROGEN HITCHHIKES ON WATER

Here’s the simple breakdown. Nitrogen travels with surface runoff in:

• Particulate form: Nitrogen attached to soil particles, decaying plant matter, or other organic material. When rain comes or surface water flow ramps up, those particles can be scraped off fields or disturbed soil and carried into nearby ditches and streams.

• Dissolved form: Nitrogen dissolved in water, appearing as nitrate (NO3-) and ammonium (NH4+). These dissolved forms can move through overland flow and infiltration, making their way into water bodies more invisibly but just as surely as the sediment-bound stuff.

Two forms, one goal: move nitrogen out of the field and into water, and then two different paths to how that nitrogen affects the aquatic environment. The reason this matters is that the two forms behave differently and respond to different management practices. If you only think about one form, you’re missing how to slow the whole system down.

WHY THIS MATTERS IN MARYLAND AND THE CHESAPEAKE BAY CONTEXT

Maryland’s landscapes—cropland, pasture, forested hills, and riparian buffers—feed a lot of runoff into streams that drain into the Chesapeake Bay. The bay’s ecosystem is sensitive to nutrient loads, and nitrogen is a big part of that story. Excess nitrogen fuels algal blooms, which can choke streams of oxygen and harm fish and other aquatic life. The Chesapeake Bay Program and Maryland’s own Department of Agriculture team up to monitor runoff, advise farmers, and promote practices that cut nitrogen entering waterways.

Understanding that nitrogen travels both as particulates and in dissolved forms helps explain why some practices work better than others in different settings. It also clarifies why timing, soil health, and landscape features like buffers all play a role. In short, you can’t manage nitrogen well by focusing on one form alone, especially in a place as diverse as Maryland.

HOW NITROGEN GETS INTO RUNOFF: THE MECHANICS BEHIND THE TRANSIT

Think of rain as nature’s washing machine. When it pours, water beads up on the soil surface, starts to move, and can pull with it whatever isn’t tied down. Here are a few key ways nitrogen ends up in surface runoff in both forms:

• Particulate nitrogen:

• Soil erosion is the big driver. When soil is bare or loosely packed, rain and overland flow pick up soil particles that have organic nitrogen or inorganic nitrogen compounds attached to them.

• Organic residues left on the surface—stale crop stubble, leaf litter, or manure on bare ground—also contribute particulate nitrogen when water momentum is strong enough to carry them away.

• Even a light, steady rain can rinse off the top layer of soil if a field hasn’t been protected by cover crops or proper residue management.

• Dissolved nitrogen:

• Fertilizers and animal waste that dissolve in water contribute nitrate and ammonium to runoff. This is especially true during or right after fertilizer application, or when rainfall events follow fertilizer or manure management activities.

• Soil chemistry matters, too. Microbes transform nitrogen between forms, and water with high moisture can move dissolved nitrogen more readily through the soil profile and into overland flow or tile drains that connect to surface waterways.

• Runoff doesn’t have to look muddy to carry dissolved nitrogen. Clear water can still transport nitrite/nitrate and ammonium downstream, quietly accumulating in streams and ponds.

In practice, these processes aren’t isolated. A single heavy rain can mobilize both particulate and dissolved nitrogen from the same field, which is why robust nutrient management needs to address both forms at once.

FROM FIELD TO WATER: THE CONSEQUENCES (AND THE SIGNS TO WATCH)

So, what happens once nitrogen hits the water? Particulate nitrogen brings sediments along for the ride, which can turbidity and sedimentation in streams. Dissolved nitrogen, especially nitrates, is a favorite for algae and aquatic plants. The bloom may be pretty at first, but it often hides a dark side: when the algae die, bacteria break them down, consuming oxygen in the water. That leads to hypoxic conditions—dead zones where fish and other critters can’t survive.

Maryland water-quality conversations often emphasize two outcomes:

• Eutrophication: nutrient over-enrichment that spurs algal growth.

• Oxygen depletion: when decaying organic matter uses up the oxygen needed by aquatic life.

The lesson here is simple but powerful: both forms contribute to nutrient loading, and both need to be addressed in practical management plans.

TURNING KNOWLEDGE INTO PRACTICAL NUTRIENT MANAGEMENT

So how do you reduce the amount of nitrogen hitting water bodies in Maryland? Here are some practical, field-friendly strategies that tackle both particulate and dissolved forms:

1. Cover crops and residue management

• Plant cover crops in fall and winter to hold soil in place, capture residual nitrogen, and reduce erosion. A living canopy protects the soil surface and slows the momentum of runoff, which means fewer particles are detached during rain events.

• Leave crop residues on the field when feasible. Stubble and mulch help shield soil from raindrop impact and reduce the amount of soil that gets detached and carried away.

2. Riparian and vegetative buffer strips

• Establish and maintain buffer zones along streams and wetlands. The grasses and forbs in these strips act like natural sponges, trapping both soil particles (particulate nitrogen) and dissolved nutrients before they reach the water.

• In Maryland, buffer programs and equivalent conservation practices are often encouraged or required in sensitive areas, helping maintain water quality downstream.

3. Timely and precise fertilizer application

• Apply nitrogen fertilizer when crops can use it most efficiently—usually during periods of active growth and when rainfall is not likely to flush it away.

• Use soil testing to fine-tune nitrogen rates. If your soil already has ample nitrogen, reduce or stagger applications to prevent excess runoff.

• Consider slow-release forms or split applications to keep nitrogen in the root zone longer and reduce peaks of dissolved nitrogen in rainfall runoff.

4. Manure management and manure utilization

• Store and handle manure carefully to limit leaching and runoff. Incorporate manure into the soil when feasible to reduce surface exposure and erosion potential.

• Plan manure spreading for drier days or when rainfall is not imminent, so dissolved and particulate nitrogen have less chance to ride away with runoff.

5. Soil health and erosion control

• Practice reduced tillage or no-till where appropriate to improve soil structure, increase infiltration, and decrease erosion.

• Use terraces, graded banks, and contour farming in sloped fields to slow water movement and give soils time to absorb nitrogen rather than carry it away.

6. Landscape-scale and monitoring approaches

• Work with local extension services (like Maryland Cooperative Extension) and state agencies to stay updated on best practices for nutrient management in your area.

• Use field water and soil testing to monitor nitrogen movement in your system. If dissolved nitrogen is high after a rainfall, it may signal the need to adjust timing or rates.

A few practical reminders for learners

• Remember the dual forms. When a question gives you options about how nitrogen is moved, the correct answer often involves both particulate and dissolved forms. If you see “only particulate” or “only dissolved,” that’s a red flag.

• Link the forms to their sources. Particulate nitrogen is often tied to erosion and organic matter, while dissolved nitrogen tends to come from fertilizer and manure. Knowing the source helps you reason through management choices.

• Tie management to outcomes. The goal isn’t just to reduce nitrogen numbers; it’s to keep streams clear, aquatic life healthy, and the Chesapeake Bay’s ecology thriving.

A Maryland-flavored digression (because context helps memory)

If you’ve ever driven along farmland near the Bay after a heavy storm, you’ve probably seen how quickly water can surge through fields and streambanks. It’s not just the rain; it’s the setup—bare soil, unprotected edges, and the way fields drain into ditches. That scenario is why buffer strips and cover crops are so central in Maryland’s land-management conversations. They’re not glamorous fixes, but they’re incredibly effective at slowing down runoff and giving nitrogen a chance to stay put long enough for plants to take it up—or for it to settle in the soil where microbes can lock it away.

A bite-sized glossary to lock in the basics

• Particulate nitrogen: nitrogen attached to soil or organic particles that can be carried away with sediment.

• Dissolved nitrogen: nitrogen present in water as nitrate or ammonium, moving with the water itself.

• Nitrate (NO3-): a common dissolved form of nitrogen that plants use, but can contribute to water quality issues when present in excess.

• Ammonium (NH4+): another dissolved form that can be taken up by plants but also vulnerable to leaching under certain soil conditions.

Putting the idea into a simple takeaway

Nitrogen moves with surface runoff in two main forms—particulate and dissolved. Both matter for water quality, especially in a state like Maryland where field runoff meets the streams that feed the Chesapeake Bay. The most effective management strategies attack both paths: protect the soil surface to cut erosion, plant cover crops to trap nitrogen, buffer water courses to filter what runs off, and time fertilizer and manure so less nitrogen is left vulnerable to rainfall. When you look at a field and an adjacent stream, you’re not just seeing land and water—you’re watching a system in action, with nitrogen on the move in more than one form.

If you’re studying this topic, keep the two-form framework in mind. It’s a simple lens that makes the whole rest of the nutrient management story much clearer: how to keep nitrogen out of water, how to protect aquatic life, and how to keep Maryland’s rivers healthy for people, wildlife, and the harvests that depend on clean water.

Would you like a quick, printable checklist that reinforces these points for field notes or a study sheet tailored to common Maryland scenarios? I can tailor a concise version focused on the two nitrogen forms, their sources, and the most practical management moves you’re likely to encounter in your courses.